In transcribing this debate about the superiority of four- and six-wheel locomotives from two issued of the 1842 Railway Times, I have used the Hathi Trust online version of the text, comparing it with the Hathi Trust’s page images. These are very accurate until the final column, the leftmost portion of which didn’t scan because in the gutter between pages. Although I have completed partly visible words when the context made the reading obvious, where it is not, I've placed my guesses in brackets. The Railway Times has few illustrations, and I have added images of the kinds of locomotives to which the writers refer. — George P. Landow

To the Editor of the Railway Times.

--It has been contended in your journal that the comparative safety of four and six-wheeled engines should be the leading feature in the present discussion, and every other matter regarded as a secondary consideration; moreover, safety was so far insisted upon, that one of your correspondents stated, “never again would his pen be employed in commendation of four-wheeled engines, if it could be shown that they were likely to lead to the destruction of his fellow creatures;” but when, unable to answer the facts and arguments advanced in proof of the superior safety possessed by six-wheeled engines, arising from the greater distance between the front and hind axles, that gentleman, with a characteristic degree of consistency, contended “that it would so much diminish the weight upon the driving wheels that the engine would be comparatively inefficient.” Now, by the above assertion we are reminded of the “ingenious arguments” which your correspondent formerly stated might be brought forward, when he contended that “to controvert facts by arguments was not the way to arrive at truth;” and his recent anxiety to make the discussion a “matter of science” rather than substantial facts gives abundance of proof that, however he may have supposed in the first instance that facts would suit his purpose, he now evidently finds them to be too numerous and inflexible for his present inclination. Again, when your correspondent was rebuking the “sneering remarks” directed against Mr. Seaward by the advocates of four-wheeled engines, he took occasion to state, that “all unseemly warmth is more derogatory to the writer than to the party against whom it is used,” but, before the conclusion of his remarks, he gave his four-wheeled friends a practical illustration of what he meant to inculcate, by characterising the six-wheeled advocates as an “unscrupulous” set; and from the concluding remarks of his letter he evidently calculates upon the amount of his own “sneering remarks” and “unseemly warmth,” to render a discontinuance of the discussion by no means “disagreeable” to the advocates of six-wheeled engines. We may therefore infer, from the progressive increase of such “unseemly warmth,” that there can be no doubt but a discontinuance of “quotations” from his own letters would by no means be “disagreeable” to your correspondent; for Mr. Hood intimates that he is under “considerable doubt” as to “the stability of his own rectitude,” and under a fear that his “ungentlemanly” letters and “poverty of argument” should establish a “very low estimate of his character.” Your correspondent also seems very anxious we should believe, that if the springs were one-third nearer together, engines would be much steadier—if so, why does he not contend for still greater stability, by concentrating the inner bearings until they are inconceivably near together, or to steady the engine upon a point—a proposition the absurdity of which surely cannot require much of the “exact sciences,” except indeed for the obvious purpose of making a plain matter of fact as unintelligible as possible? Moreover, Mr. Seaward's remarks upon that point referred not exclusively to defects in the road, as intimated by Mr. Hood, but also to “defects in the engine itself,” and to any person willing to understand Mr. Seaward, his remarks were too plain to be misunderstood, and too firmly grounded upon “numerous facts” to be refuted on that point. Liverpool Standard of Friday last, and understanding It was stated by Mr. Hood, that the arguments from our Chairman that Mr. Boothby (who moved of Mr. whishaw would do equally well to illustrate “the expansive working of steam, the combustion of smoke, the toll question on railways, or the suppression of Sunday travelling.” However on some of his general arguments upon the superior twisting powers of four-wheeled engines being brought forward, to prove that they are too susceptible of twisting &c., he asserts that his arguments have been “perverted from their legitimate meaning.” Now, the “legitimate meaning” of your correspondent appears to be that four-wheeled engines, from their shortness, will twist about wherever he desires them, but any twisting about from the same shortness, without his permission, he considers is not “legitimate,” and so far I agree with him.

Left: Robert Stephenson's The "North Star" — 2-2-2, 1837: Great Western Railway (seven-foot gauge). Although the North Star had only a single set of driving wheels, the present of the other two axels might make this an example of a six-wheel locomotive. Right: Edward Bury's (4-wheeled) 2-2-0 Passenger Engine, 1837: London and Birmingham Railway. [Click on images to enlarge them.]

Mr. Hood some time ago stated, that only nine crank axles have been known to fail in about 250 four-wheeled engines during four years. Now, if that statement be correct, it would appear that their failure of late must have been very rapid, for upon one of those lines alone it is stated, in a contemporary journal, that “during the last two years there have been ten cases of fractured crank axles;” hence the four-wheeled engine makers do not appear to be infallible on that point, although they have had the protection of inside bearings, but until better dimensions and proportions are given to the cranks of both four and six-wheeled engines, I shall not be surprised at hearing of a hundred broken cranks every few years.

As to your correspondent contending that the cylinders of the Liverpool were both “horizontal” and at an “angle,” and such like observations on the early engines, they are “not even worth the trouble of contradiction.” Mr. H. also attempts to shuffle out of the superior disposition of four-wheeled engines to rear up in cases of collisions, but when we recollect that he has again and again admitted their greater liability to “pitch” I scarcely need attempt a definition of such synonymous terms as pitching and rearing.

The "Derwent," a six-wheel engine dating from 1839: Stockton and Darlington Railway. [Click on images to enlarge them.]

Before giving additional cases in proof of the superior agility of four-wheeled engines in “rearing” and twisting motions, I beg to state that the observations of Mr. H., on the Liver, and Samson, or Goliath, abound with his usual inaccuracies. The Clarence, one of “the present excellent four-wheeled engines” on the Leeds and Manchester line, ran into a train of waggons, and the engine, although the heavier body, reared upon the last waggon, and being more readily twisted, “in proportion to its shortness,” she took off on the inside of the curve, and set fire to the waggon in the display of her “rearing” powers. Nos. 72 and 73. Mr. Cruckshank, during the discussion, cited two cases of four-wheeled engines running off the road at curves, whilst the six-wheeled engines kept on the same road, on the same day, both before and after “the man traps” pitched themselves off the rails. “Seeing the internal vexation” which the mention of an Eastern Counties' case has previously produced, “I would not willingly add to such a corroding passion by any further remarks, were it not necessary to justify” six-wheeled engines from the “unfounded assertions” of your correspondent; I therefore hope that the mention of the following case will not be productive of another exhibition of that nature.

No. 74. I am informed that an engineman of the name of Meadows, and Leech, his fireman, both lost their lives by one of the Eastern Counties' four-wheeled engines “pitching” herself off the road. Under the force of quotations from his own letters, it is not a little amusing to see the anxiety exhibited by Mr. H. for a temperate discussion of the subject by six-wheeled advocates; indeed, the solicitude of your correspondent to get out of the predicament into which he has placed himself through the unguarded style of his writing is such, that about two columns out of three in his last letter are occupied, not with the subject under discussion, but upon “absurdities,” “unscrupulousness,” “splenetic anger,” and in fact upon anything rather than the subject of four and six-wheeled engines, and as to some of the knotty parts of the question, if answered; your corespondent has stated, as a proof of his anxiety to discuss the subject “in all its bearings,” that it “would require more space than it is worth.”

Once for all I beg to state, that if the gentlemen whose letters have appeared under the signature of “Charles Hood,” are smarting as much as they appear to be under the lash of quotations from their own letters, it ought to be sufficient to convince them of the “indiscretion” of putting such weapons into the hands of their opponents; they will joke, and yet complain of a breach of etiquette at a verbatum return of the compliment. If less safety could be traced out in six-wheeled engines than others, or that they had been so severe on the lives and limbs of our fellow-countrymen as the four-wheeled engines have evidently been, never again would I appear as a six-wheeled advocate, if even it were true that safety could only be obtained at the expense of being “comparatively inefficient.”

I am, Sir, your obedient servant,
Hull, February 21, 1842.

[From the issue of 1842]

To the Editor of the Railway Times.


The Report of the officers of the railway department, lately presented to Parliament, contans, as might naturally be expected, some importart information on the subject of four-wheeled and six-wheeled engines; but it is worthy of remark, that, among the large number of returns made by the different Companies, very few of them state any definite reasons for the adoption of the peculiar form of engine which they employ. This is partirularly the case with lines which employ six-wheeled engines, for, of thirty-four Companies using this form of engine, only six have given any tangible reasons for their preference of this plan. The subject, indeed, is one on which Railway Companies may well be excused entering into argumentative discussions, as well from the abstract difficulty of the subject as, also, because it is by no means likely that on such a question there will be perfect unanimity of opinion among the different members composing the administrative and executive departments of a Railway Company. Not only, indeed, may the process of ratiocination vary, by which diferent persons arrive at the same conclusion, but it must frequently happen that persons whose opinions at one time strongly influenced the decisions of a Company on such a subject, may subsequently cease to take part in their deliberations, and their successors can of course be in no way answerable for the opinions of those who preceded them. In those cases, however, in which reasons have been given in the returns alluded to, for the adoption of particular forms of engines, the arguments may, I think, very fairly be made the subject of examination, the same as any other public document; and I therefore propose to offer a few remarks on some of the reasons given for the use of six-wheeled engines.

Left: A later four-wheel engine: The London and South-Western Railway 580 — a 2-4-0 express passenger engine. Designer: W. Dean. Source: Pattinson, British Railways (1893). Right: A 2-4-0 built by Beyer Peacock & Co., Manchester. National Railway Museum, Utrecht, the Netherlands. [Click on images to enlarge them.]

The French have a proverb which is highly characteristic of their natural habits, Contredire c'est quelquefois frapper à une porte pour sapoir, s'il y à quelqu'un dans la maison: - (“To contradict means, sometimes, to knock at a door to know whether anybody is at home.") They do not consider contradiction to a man's opinion necessarily involves personal opposition, but that it merely expresses the desire to examine the relevancy of his arguments, and to discuss the subject with him. As some of my former letters on this question have been misunderstood in this respect, I am the more desirous to give this explanation of my reasons for the remarks I am about to make—because, if my observations were considered to arise from opposition, when I was merely defending my own opinions against objections which were raised against them, it is much more probable that I might be misunderstood, without such explanation in the present instance.

“Coppernob” 0-4-0. No. 3. Furness Railway steam locomotive. 1846. Photograph and text courtesy of the National Railway Museum

Of the six Railway Companies which have given reasons for employing six-wheeled engines, only one or two have given anything besides a few general observations on the subject. The Manchester and Birmingham Company, in fact, is the only one which has gone fully into the reasons of their Preference of six-wheeled engines; Mr. Buck, the Company's Engineer, having given an elaborate Report on the occasion. The reasons given for this preference I will now examine, first giving the arguments of Mr. Buck, and then remarking upon them seriatim.

1st. “The engines with six-wheels are of much greater length; hence it is evident that, in passing an inequality in the level of the rails, the angle of ascent and descent will be diminished inversely as the distance between the fore and hind axles; and because this distance in the ordinary four-wheeled engines is from 5 feet 9 inches to 6 feet, and between those of the best six-wheeled engines is 12 feet; this angle will consequently be diminished one-half. Thus we see, that in passing over an inequality, both engines running at same velocity, that which is only half the length of the other will pass over it in half the time, the angle of ascent and descent being double ; hence the shock is four times greater in the short engine than in the long one.”

In this reasoning the vertical pitching of the engines is assumed to arise solely from the angular deviations from a true horizontal line which the engine would form when measured simply in a statical [sic] position. This, however, is not the case; for the angle which any engine would form with a true horizontal line under these circumstances, would be far too small to be in any degree appreciable as a practical result. By the term “inequality in the level of the rails,” I understand some imperfection at the joints of the rails, by which one rail may be a trifle higher than the other. If this imperfection amount to one-eighth of an inch, the angle which an engine would form when the points of support are 12 feet apart would be less than one-twentieth of a degree. But if the “inequality in the level of the rails” should arise from unequal sinking of the sleepers, and this should amount to one inch under one wheel more than under the other, then the angle with the same engine as before supposed would amount to about four-tenths of a degree — in either case, the deviation being far too small to be at all appreciable. All, therefore, that follows, about the shock being four times as great with a short engine as with a long one, necessarily falls to the ground, because the argument is based upon the measurement of the angle when the engine is considered as a statical body, instead of considering the whole question as a dynamical effect.

Almost six decades after the present article, Sharp Stewart &. Co. manufactured this six-wheel (0-6-0T) engine no. 1257 for the Great Northern Railway. 1899. Right: North East Railway 0-6-0 No. 1275 (1001 class). 1874. Photographs courtesy of the National Railway Museum. [Click on images to enlarge them.]

The pitching motion of any engine would be almost inappreciable if measured in the way here stated. It however in reality arises almost entirely from a far more powerful cause, namely, the striking of the wheel of the engine against any inequality existing at the joints of the rails; the force of the blow being the momentum of the engine, and the angle of deflection being equal to the angle of incidence which the whole mass of the engines, supposed to be concentrated into its centre of gravity, forms with the impediment against which it strikes. This angle will be rather greater in four-wheeled than in six-wheeled engines, and consequently it has never been denied that the vertical pitching motion is greater in the former than in the latter: but the difference is neither so great in extent as, nor does it arise from the cause which, Mr. Buck has assigned. In a six-wheeled engine the angle which the centre of gravity of the whole mass of the engine would form with an impediment in the joints of the rails such as we have here supposed, would be about 40 degrees; while we have seen that the angle of Mr. Buck's hypothesis would only amount to one-twentieth of a degree; and the effect produced is probably in about the same proportion. The effect of this deflection at so high an angle is (as I have already explained in a former letter) counteracted by the force of gravity, otherwise the pitching motion would be enormous.

2nd. “If the four-wheeled engine be of equal power with the six-wheeled engine, and of a proportionate strength, its weight will be such that there must be a greater load upon a single pair of wheels than in the six-wheeled engine, and consequently a greater deflection of the rails and depression of the road.”

By reference to the returns made by the several Companies to the Board of Trade, it will be seen that this statement is founded in error; for, taking any specific sized engine — say, for instance, 13 inch cylinders — the weight on the driving wheels will be as nearly as possible the same in both classes of engines, as appears by the returns from the Brighton, the London and Birmingham, the Liverpool and Manchester, and some other Companies, which have given the particulars and weights of their engines.

3rd. “A four-wheeled engine, like a four-legged stool, if placed upon a surface which is not a perfect plane (as upon rails not in good order) will be supported by two points only, namely, by one fore and one hind wheel in the direction of the diagonal, leaving it at liberty to rock sideways; and supposing the distance between the axles to be 6 feet, the rails being laid at the ordinary gauge, it will rock upon the diagonal of 7 3/4 feet long. But the amount of inequality which occurs in the rails, in the distance between the fore and hind axles of a four-wheeled engine, has, in the case of the six-wheeled engine, to be traversed by another succeeding pair of wheels, fixed in the same frame. The rocking is thus retrained, and almost entirely prevented, the strain being transferred from the first diagonal to the second.”

On what authority this diagonal rocking is supposed to take place I am not aware; but it is evidently based on two erroneous suppositions, namely, the entire inelasticity of the engine, and its statical condition; instead of, as in reality is the case, its perfect elasticity and its varied dynamical effects. The rocking motion here described is impossible to occur as a primary motion; and if it have any existence whatever, it cannot arise from the cause assigned, but can only exist as the resultant of the vertical pitching motion and the transverse oscillation. We will suppose, however, that a four-wheeled engine is resting on the two wheels diagonally, as above stated, and that the other two wheels do not touch the rails, in consequence of the rails being one inch lower at one part than at the other, which, according to the above theory, would then cause the engine to rock diagonally. Now, the force which would cause the rocking is evidently the effect of gravity acting on the heavier of the two ends thus unsupported; and the velocity with which it would descend through the supposed distance of one inch (or '0833 of a foot) would be w/(16 x .0833)= 4 x .2886= 1.154 feet per second, supposing it to be acted upon by the ordinary force of falling bodies. But it will not descend with this velocity, for its gravitating power will be only that which would be due to the descent of the heavier of two unequal weights suspended over a pully by a cord. Now, if the engine weigh ten tons, and we imagine one-half the weight to be thus momentarily unsupported, and that the oscillating ends are of the proportional weights of 2 1/4 tons and 2 3/4 tons respectively, the velocity of descent will be obtained by multiplying the above velocity by the difference of these numbers, divided by the sum of the same numbers, or

x -10 = 115 feet per second. The time of descent of the heaviest unsupported ends of the engine, through a space of one inch, will therefore be 1/72 of a second; and if we suppose the engine to be travelling at the rate of 25 miles per hour, it will in this period of time have passed over a space of rather more than 26 feet. It is evident, therefore, before the smallest possible amount of rocking can occur in this way, the engine will have so completely changed its position on the rails that the whole of the conditional circumstances will be altered, and the effect neutralised. It is almost unnecessary to remark further, that the exemption from this rocking motion, claimed for six-wheeled engines in consequence of “the strain being transferred from the first diagonal to the second,” is equally erroneous. No transfer of motion can take place in this way without reaction; and therefore, if this transfer could take place, as supposed, the two diagonals must be moving in opposite directions at the same time, and with equal force, thus producing a double motion in the engine.

4th. “The thrust and pull of the connecting rods have a tendency in every part of a revolution, except at the moment when they are in a horizontal position, to lift the fore wheels off the rails, and they act with variable intensity on each side of the machine alternately, tending to produce a rocking motion.”

The effect of the connecting rods in producing a rocking motion (or, more properly, an undulating motion), must be greater in proportion as they are more distant from the centre line, or axis, of the engine; and as the distance of the connecting rods from the centre of the engine is only 10 inches in the four-wheeled engine, while it is 20 inches in the [371/372] sixwheeled engine, it is clear that the relative advantages of the two classes of engines are exactly the reverse of what is here implied.

5th. “The driving wheels of the four-wheeled engines are necessarily behind, and there results the same objection to this mode of applying the motive power to the machine itself as there is to putting the engine behind the train to propel it.” These two cases can bear no possible analogy, because in one case the moving body consists of a single machine of which all the parts must move in unison—in the other the moving body consists of a great number of separate portions, not so connected as to ensure their motions being in harmony with each other, but on the contrary, each part having a motion of its own, distinct from that of the others by which means the motion of one part which happens to be rather heavier, frequently increase, the motion of another part which happens to be lighter, to a very dangerous extent; an effect which could not take place if the train could be so connected as to form absolutely one body, of which all the motions were performed simultaneously and in equal times, as is the case with a locomotive engine.

6th. “Engines with four-wheels when running at high velocities are subject to a violent horizontal action, tending to beat the rails outwards, and communicating a similar motion to the tender and train behind. This is produced by the following causes, namely, the driving wheels (in both classes of engines) do not, and cannot, act at each instant, with equal intensity to propel the machine forwards, and this inequality of effort of the wheel upon the rails arises and is produced as follows: First, the cranks of the two cylinders pass their respective centres at different times, so that when one connecting rod is reversing its motion, the other is exerting its maximum effect; this, together with the small amount of torsion in the crank axle, and play of the joints and bearings, causes one to cease to act for a moment. Again, the tires of the wheels being frustra of cones, the lateral movement of the engine between the rails causes first one wheel and then the other to be alternately running upon a larger circumference of its tire than its fellow ; when thus acting, the vertical radius of one driving wheel is shorter than that of the other, and consequently the intensity of its action is greater. In six-wheeled engines the driving wheels are in the centre of their length, hence their horizontal vibration is restrained by the leading and following wheels.”

The above-mentioned effect of four-wheeled engines “to beat the rails outwards” might very properly have been stated as a general result from all engines, and not as a peculiarity of those with only four-wheels. The first reason that is given is, singularly enough, stated to arise from a cause of action common to “both classes of engines,” while the effect is attributed to only one class — namely, the four-wheeled engines. Now, the action here described is the same as that already answered in objection 4; where it has been shown that the action is in reality greater in six-wheeled than in four-wheeled engines. The second reason given for this beating of the rails outwards is also common to both classes of engines, and its amount will depend upon the relative amount of play which is allowed between the flanges of the wheels and the rails. If the engines be respectively 12 feet and 6 feet between the axles, and the play of the wheels be the same in both, the angle formed with the rails when the transverse horizontal motion occurs, will be double in the latter case to what it is in the former. But as the play is not the same, but is generally 1 inch for six-wheeled engines and 3/4 inch (and very often less) for four-wheeled engines, the angles will be relatively as 2 to 3, instead of as 2 to 4, which would be the case if both had the same play. Now the force of the blow on the rails is directly as the momentum, and the sine of the angle of incidence; but as the velocity is the same in both classes of engines, the force of the blow will be simply as the weight multiplied by the sine of the angle of incidence; and therefore as the weight of the six-wheeled engine is about 15 tons, and that of the four-wheeled engines about 10 tons, we shall, have 15 x 2 =30 for the force of the blow from the six-wheeled engine, and 10 x 3 = 30 for that of the four-wheeled engine — or the effect is precisely the same in both engines, when the road is perfectly straight.* The effect on curves we shall see immediately.

7th. “It is alleged by some that four-wheeled engines pass through curves easier than six-wheeled engines do, and are less liable in such places to mount the rails. Now, as this seems very plausible, and theoretically correct, it appears to have been generally admitted as indisputable; but on a close examination of all the circumstances of the case as they actually occur in practice, the contrary will be found to be true; at least it will be so in reference to all curves of such radii as are admissible on railways. To be convinced of this, it will be necessary to refer to a mathematical investigation, which I have given as an appendix to this Report, by which it is found that, with the usual allowance for play between the flanges of the wheels, an engine whose extreme axles are 12 feet asunder, and another whose axles are six feet asunder, will pass through a curve of 576 feet radius, with equal resistance, or with equal facility; and it also appears, from the same investigation, that upon all curves of a larger radius, the longer engine has the advantage.”

The whole of the above argument is based upon the investigation which is alluded to, and which is appended to the Report in question. This mathematical demonstration (although it is not so stated in the Report) is understood to be the production of Mr. Barlow, junior, the son of the much-esteemed Professor at Woolwich, and it possesses all the merit which might be naturally expected, coming from such a quarter, and treating on such a subject. But although Mr. Barlow may have furnished this demonstration, I do not imagine that he is answerable for its application. It is an excellent exposition of a trigonometrical question; but it could never have been intended as a theorem for illustrating a question relating to bodies in motion. Assuming, as is done in this theorem, that of two engines (which have equal play between the flanges of the wheels and the rails) the one is double the length of the other between the axles, there must be a certain sized curve compared with the length of the engines, where both will form the same angle with a tangent of that curve; and this Mr. Barlow has proved, with 6 feet and 12 feet bearings of the engines, and 3/4-inch play of the wheels, to be 576 feet. But it must be remembered this is a mere statical position. The force with which the flanges of the wheels press against the rails, we have already seen, is (with equal velocities) directly as the weight of the engines; and therefore this will be in the proportion as 10 to 15, with the four-wheeled and six-wheeled engines respectively. But, in addition to this, the time required to bend any body from its direct course is as the square root of its length; which element again becomes an additional measure of force in the present case, the time being directly as the force. The square roots of the length of the engines will be 2.45 and 3.45; therefore 10 x 2.45 = 245., and 15 x 3.45 = 51.75; or the relative force and tendency of the two engines to mount the rails will be as 24 to 51 —the six-wheeled engine having more than twice the tendency which the four-wheeled engine has to over-ride the rails, even under these, the most favourable circumstances which can be produced, and frequently the difference will be double this amount.

8th. “If an axle of a four-wheeled engine break, the machine must fall; but on the contrary, if an axle of a six-wheeled engine break, it will not fall; and to ensure additional safety, the middle or driving wheels of a six-wheeled engine should not be without flanges, in order that, in the event of one of the axles breaking, the engine may not leave the rails.”

This assertion is singularly erroneous; as no instance can be produced where a four-wheeled engine has fallen, or run off the rails in consequence of a broken axle. In every instance where a four-wheeled engine has broken the crank axle, it has taken the train in safety to the next station; and it has even occurred that an engine has conveyed a train a distance of seven miles after the fracture of the crank axle. When such an accident happens to a six-wheeled engine with outside bearings, it becomes completely disabled, and the great liability of this form of engine to such accidents affords abundant opportunities for verifying this result.

The above remarks contain, I believe, the whole of what is important in Mr. Buck's Report, as the remaining observations he classes under the head of imperfections, which he considers are not necessarily consequent upon the form of the engine. In the above extracts, I have given the whole of Mr. Buck's remarks without abridgement; excepting some objections which he makes to the weight of the fire-box overhanging the wheels too much in the four-wheeled engine. This observation, I consider, only applies to improperly constructed engines, and I have therefore omitted it, as I fear I have already exceeded reasonable limits in the length of this letter. I considered it necessary however to give the whole of Mr. Buck's reasoning before commenting upon it, as otherwise the text and the context, by being separated, might convey a different meaning to that intended. Although these arguments have, I think, been shewn to be entirely erroneous, they are by far the most ingenious of any that have been brought forward by the advocates of six-wheeled engines.

In addition to the reasons given by Mr. Buck on behalf of the Manchester and Birmingham Railway Company, there are five other Companies, namely, the Croydon, Dublin and Kingstown, Liverpool and Manchester, Hull and Selby, and Great Western, which have given reasons for adopting six-wheeled engines. In all, however, except those given by Mr. Gibbs in behalf of the Croydon Railway, there is nothing which is new, or which has not been amply discussed in the course of the letters which have already appeared in your journal. I will, therefore, merely make a few remarks on the reasons Mr. Gibbs has assigned for his opinion.

Mr. Gibbs states that his objections are principally when four-wheeled engines are used on railways not sufficiently consolidated, and possessing steep gradients; for “to keep time throughout the journey, a high degree of velocity must be attained in descending, which is often on a new road carried to the limit of safety. Now, it is clear that if the engineman shuts off, or even diminishes, his supply of steam, the engine, from its greater friction, will diminish its speed much earlier than the train, consequently the train will become for a short time the propeller of the engine. Now, imagine the engine having the largest driving wheels and boiler the case will admit of (and which is necessary for the gradients on such a line as I have alluded to), and that a large portion of the weight of the engine is placed on the driving wheels; also, suppose the road to be new, and consequently, undulating even very slightly; again, suppose the limit of velocity attained in descending, to make up the loss of speed in ascending the summit. You will now perceive a probable position; if the engineman suddenly shuts off or even diminishes his steam, the previous oscillation will be increased until the engine tumbles over, not sideways, but in a line diagonal to its length. * * * The danger arises from the engine being supported on points and having too much overhanging [right?] at on one side of the driving wheels, which is [needed?] to obtain the necessary traction. I never [made] the same objection to low four-wheeled engines [as?] the wheels are coupled and of one dimension, nor I ever apprehend danger from the small [?] four-wheels, such as are used on the [Gr]enwich Railway. * * * It may be said a six-wheeled engine will present the same cross [actio]n; but as I before stated, the oversetting of - engine under the circumstances I have named not sideways, but angularly, somewhere in a line . . .from corner to corner. Now, if you make a as section in this direction, and find the centre of gravity, you at once perceive the greater danger [over?]ersetting of the four-wheeled engine as compared with that of six-wheels.”

Now the above, which constitutes the whole of what is important in Mr. Gibbs's Report, rests upon an assumed fact, for which neither practical nor theoretical reasons are given. All that is stated about the train becoming the propeller of the engine in descending steep gradients, when the steam is suddenly shut off, is perfectly correct; but why the previous oscillation of the engine should, under the circumstances, be increased, I cannot imagine. The oscillation of the engine arises from the action the pistons, the play of the springs, and the conditions of the road, or the unevenness of the rails, mnchromising together; but if the steam be diminished, and the speed be thereby lessened, this undulation ceases, and the oscillation gradually diminishes. Again; the oscillation which is thus [augm?]ented (as Mr. Gibbs states) causes the engine “to tumble over, not sideways, but in a line diagonal to its length.” Now here is an opinion where substantial reasons would have been peculiarly valuable. All opinions ought to be founded either on exrience or on substantial arguments, otherwise they can be of but little value. As regards the [fact?] of engines tumbling over diagonally, I am not sure that any evidence exists; nor is it at all probable there should be, because such an accident must happen so instantaneously that no observations can be made. How, then, stands the theoretical consideration? The reason assigned for this probable effect is, in consequence of the points of support being too near together. But as the diagomal compared with the central line of the engine is nearly in the proportion of 8 to 6, so also in this proportion must be the probability of its pitching over in one direction rather than another. But supposing it could possibly pitch over diagonally, what is it that determines whether that diagonal shall be from left to right, or from right to left? For every effect there must be an adequate cause; and therefore if engines are liable, under the circumstances stated, to pitch over diagonally, there must be a cause why they should pitch over on their longer axis rather than on the shorter; and also why they should pitch over on the right hand in preference to the left, or on the left in preference to the right. Until satisfactory reasons can be assigned, or facts adduced, in support of these opimons, even the high talents of Mr. Gibbs as an engineer can give very little importance to mere vague assertions which appear contrary to all common experience.

During the course of the letters which have appeared in your journal on the relative merits of four-wheeled and six-wheeled engines, a great number of very interesting questions have been debated; and I propose, in a subsequent letter, to give a short abstract of the principal arguments which have been brought forward, in order to show how several opinions have been either confuted of proved.

I am, Sir, your obedient servant,

East-street, Blackfriars, March 21, 1842.

* The sine of the angle has here been assumed to be as the angle, which, on account of the smallness of the angle, may be done without sensible error. When the play of the wheels is 1 inch, the six-wheeled engine will form an angle with the rails of only 1/397 of a degree; and the four-wheeled engine, with the play of three-quarters of an inch, will form an angle of 1/596 of a degree. The sines of these angles will be .006923, and .010384, respectively; numbers which scarcely differ from the proportions of 2 and 3. We may likewise calculate the force of the blow upon the rails which tends to beat them onwards. The force of the blow given by a weight of 1 lb. moving at the rate of 5.673 feet per second, is equal to 15-143 lbs., and the force varies as the square of the velocity. Therefore, supposing a six-wheeled engine weighs 15 tons, and moves at the rate of 25 miles per hour, we shall have 5.6732 : 15-143 :: 36.666.2 : 6320leave the ra5; and, therefore, 632.5 x 15 x 0.06923 = 65-68 tons as the force with which it tends to beat the rails onwards, under the circumstances here assumed.


“Four-Wheeled and Six-Wheeled Engines” Railway Times. 5 (21 February 1842): 352-53, 371-73. Hathi Trust online version of a copy in the University of Michigan Library. Web. 2 September 2018..

Last modified 2 September 2018