The following explanation of weirs for damming, fishing, and other purposes and their development into locks on river and canals comes from H. R. Robertson's Life on the Upper Thames (1875). Since much of Robertson's book covers life and sport on the Thames, particularly fishing, he introduces weirs by pointing out that although they unfortunately rid the river of salmon, they encourage the growth of trout. He next cites the mention of weirs in literature from Chaucer to his day and then explains the development of weirs into river (and canal) locks. I have omitted most of the quoted poetry, in part because Robertson often focuses on the misuse of terms. In what follows I have both added headings and omitted long passages. [Click on images both to enlarge them and obtain additional information.] George P. Landow.
NO invention, however great an improvement it may be, ever seems to bring about a state of things better in all points than that which it supersedes. Accordingly there are reasons why inland waters, as a medium of conveyance, are in many cases preferable to railways. They are especially adapted for those goods which are very heavy, very bulky, or which cannot well bear any rough carriage. For the reason last mentioned, bricks are, if possible, always transported by water; it being found, from the smooth and easy motion of a boat, that the load is seldom damaged, while by rail the percentage of bricks that get broken is very large. To those persons residing near a river the expense of sending goods by it is frequently less than by any other mode of conveyance.
Two ways of crossing a river without bridges. Left: The Ford. Right: A rope-ferry.
Inland navigation by means of rivers and canals is obviously at a disadvantage when compared with the road or the rail as regards rapidity of transit. The decided preference that rivers seem always to manifest for a circuitous route often renders the distance between two towns on the banks half as much again as the direct road between them. Besides, the regularity of the water-traffic is liable to be interfered with by drought in the summer, and floods or frost in the winter. It is no wonder, then, that the railway should have drawn away most of the traffic from the Thames. The towing path along the side of the river was formerly valuable property, certain farms having a prescriptive right to supply the use of horses to the barges while passing. In one instance, to our personal knowledge, a path of this description which twenty years ago realised £200 a year, now scarcely repays the expenses of keeping in repair.
The Weir as a solution to the problem of shallow, fast-moving water in a river
Left: Hart's Weir. Right: Weir paddles -- the equivalent of gates on a modern dam. Both images from the Halls' The Book of the Thames from its Rise to its Fall.
The chief difficulties that exist in the navigation of rivers are owing to the irregularity in the depth of the stream at different places, and the varying velocity of the current The great obstacle, therefore, to be surmounted may be described as a shallow extending the whole width of the stream with a considerable rush or fall of water over it. This state of things naturally occurs with greater frequency the farther one penetrates inland towards the source of a river.
The most primitive way of overcoming the difficulty has been to erect a movable dam all across the river, below the shallow; the boards of the dam being, of course, high enough to keep back sufficient water to enable a boat to float over the shallow. By this means a boat descending the stream meets with no impediment till it reaches the dam, or "weir" (pronounced "wire" by the riverside people), as it is technically called. The boards composing the dam are then removed, and the boat proceeds for some time with great rapidity, owing to the increased volume of water by which it is carried along. The temporary depth thus produced while the body of water descends enables the boat to pass over many shallows below the weir. This removal of the boards is called "flashing" a weir, and is "the tide in the affairs" of bargemen, the neglect of which lands them "in shallows and in miseries." Of course it is in the summer and autumn that these artificial aids to navigation are most employed, there being at other times enough and to spare of the then precious fluid. We first thought that the word "flashing" was a vulgar corruption from "flushing," but as it appears in the printed orders of the Thames Conservancy we suppose it is correct. The suddenness with which the pent-up water rushes away, and its glitter and white foam, may not improbably have suggested the word. When the water is low, the river is flashed twice a week by the regularly appointed keepers of the weirs, each of them waiting till the water from the weir next up the stream has reached him. By this means a continually augmenting volume of water descends, on the flood of which the whole of the traffic is carried. Sometimes the bargemen are sorely tempted to draw a flash on their own account, when they may have been unusually delayed, or are from any reason particularly anxious to proceed. However, the Thames Conservators are severe, and have issued handbills stating that all persons ofiending in the above case render themselves liable to a penalty of;^ 20, and the strict observance of the regulations is considered so essential that the prosecution of ofienders is deemed by them an imperative duty.
The parts of a simple weir
The different parts of the most simple weir are first the sill or fixed beam, laid securely across the bottom of the stream; then, directly over this, but considerably above the surface of the water, is placed a second but movable beam. Against and in front of these parallel beams a set of loose boards is placed upright and close together like a door. These loose boards are called paddles, and the long handles with which they are ftimished rest against the upper beam, the pressure of the stream serving to hold them in their places. Between the paddles are placed upright supports termed "rimers; " and when a second set of paddles is employed over the first to obtain a greater depth of water, this set is called the "overfall."
Uses and effects of a weir (1): waterpower
Two watercolors by Mortimer Menpes of the Thames: Left: Hambleden with its weir. Right: Streatley Mill, whose waterpower comes from a weir.
A weir, though constructed for the purpose of facilitating the navigation, is incidentally of considerable use in other ways. The damming up of the water renders any side stream that may happen to leave the main current above and rejoin it below a weir available for turning a water-wheel; consequently we find a mill of frequent occurrence in its neighbourhood. The picturesque appearance of the spot is thus often greatly enhanced, for if the miller's dwelling should chance to be an old building, it is sure to be pretty; if a new one, I am afraid we must say it is pretty sure not to be so.
Uses and effects of a weir (2): functions as a bridge
Two kinds of weirs that also function as bridges: movable and fixed forms.
As the largest barge is far from occupying the full width of the stream, it is practically found that only a portion of the bridge is required to be movable. In our illustration to this chapter the man who is putting down the paddles is standing on the movable part, called the "swing-bridge." It revolves on a pivot close to the edge of the water, and the weight is balanced by the increased thickness of the beam at the landweird end, on which is often placed a great stone or other heavy substance. The upper beam and hand-rail across that part are, of course, removed before the bridge is swung round, and it is for this purpose that the two handles which may be noticed are added. . . .
Uses and effects of a weir (3): provides a fixed bridge
"Shall Thames be barred its coarse "with stops and locks.
With mils, and hils, and gravell beds and rocks,
With weares, and weeds, and forced Hands made,
To spoil a publike for a private trade?" — John Taylor (the Water Poet, 1640).
OUR explanations of the preceding drawing apply in a great measure to this, modified, as the name implies, by the fact that in this case the whole structure is permanent. Thus, instead of paddles with long handles that are removed bodily, we have them here made to slide in grooves. They are raised by means of the chains which are coiled round axles placed just below the upper beam. The axles are caused to revolve by inserting 'nto them a staff with a square end, for which purpose the square holes are made that may be observed near either extremity of the axles. A short chain, suspended from the upper beam and finishing with a hook, is used to retain the paddle at whatever height may be thought desirable, by attaching the hook to a link in the chain first, alluded to. Some of the paddles are represented as left down, so that the mode of raising them may be the more readily understood by noticing the different positions of the chains in either case.
One of the incidental uses of these weirs is that the framework erected may be with very little trouble utilised as a bridge. In the thinly populated districts of the Upper Thames regular bridges are few and far between, so that these slight foot-bridges save the poor people many a weary mile in their walk to the nearest market-town.
The noisy rush of water that continues for an hour or so after the flash is drawn is enough to terrify a child, for whom the railing is at too great a height to be much protection. There is a considerable trembling of the old timber, with a tumble-down air pervading the whole thing, that may well justify the timidity of the little girl we sketched while being carried over by her father, and looking the picture of alarm.
One of the effects of sending down the head of water is to cause the big trout to show himself at the surface, rising first at one part of the pool and then at another; but, as we believe, more in wantonness than for food. We fancy it is his way of testifying that the boiling and eddying state of the water is his idea of the correct thing in the way of a trout-stream, and a protest against man's endeavour to improve the river to a dead level. Visitors to the Crystal Palace or Brighton Aquarium will have noticed how fish of many kinds seem to revel in the bath of air-bubbles that enters with their fresh supply of water. Mention of the trout reminds us that one paddle is frequently left up when the rest are down, for the sake of putting a net in the passage thus made, in which any fish carried down by the stream or trying to descend may be entrapped. As this description of weir is a permanent structure, provision is made for the passage of boats by means of an ingenious arrangement called a " lock," which is described and explained in our next chapter. . . .
Pre-nineteenth-century confusion of terms “weir” and “lock”
Before even the invention of what we now call a "lock," the word was common enough, and is found in many old authors when speaking about the river. The context invariably shows that it was used for what, at the present day, is called a "weir;" . . . . Chamberlain's "Survey of London," published in the year 1770, mentions the existence of many "locks" on the Thames, which are thus defined: "Machines of wood placed across the river, and so contrived as to confine the current of water as long as is found convenient — that is, till the water rises to such a height as to allow depth enough for the barges to pass over the shallows; which, being effected, the water is set at liberty, and the loaded vessel proceeds on its voyage, till another shoal requires the same contrivance to carry it forward." This, it will be seen, answers precisely to what we have defined a "weir" to be. At that date the expense to a barge for passing through all these weirs amounted to nearly fourteen pounds. This was, however, only during the summer, when the water was low, these weirs at all other seasons being  removed; and the same authority adds that "from London Bridge to Bolter's Lock, which is a distance of fifty-one miles and a half, there is no lock on the river."
Opening a lock
Two locks on the Thames: Osney Lock Left: The Ford. Right: Lechlade seen from the First Lock.
That, drawn off sideways, smooth and still,
The pent-up flood may go
To where the lock doth fall and fill,
"With gate-checked ebb and flow.
"Like subtle counsel, that supplies
A safe and sidelong way
To round whatever barriers rise
Across the forthright way." — Tom Taylor.
LOCK, or pound, as it is sometimes called, is an enclosure between two pairs of gates, and is usually large enough to admit several barges at the same time. It is the necessary accompaniment of the fixed weir, alongside of which it issometimes placed, though more frequently on a side-stream, or "cut." The level of the water above and below the lock corresponds with that above and below the weir; but in the lock itself the water level can be varied at pleasure, between the two extremes, by means of valves in the gates. These permit the water to enter through the upper gates and to escape through the lower ones. When it is necessary to pass a boat upwards through the lock, she is first floated in at the lower gates, previously opened, and which are next to be shut. Water is then admitted through the valves of the upper gates till it has filled the lock-chamber to the level of the water above the weir, and has, of course, raised the boat along with it. The reverse of this process will obviously conduct a boat down through the lock, which is said to be empty when the water in it is at the lower level, although it has still the same depth of water as the lower river.
Left to right: (a) Old Windsor Locks. (b) Sunbury Locks. (c) Teddington Locks.
The tendency of the age to substitute the mechanical and the ugly for the simple and picturesque is noticeable on the Thames as well as everywhere else. Hideous turret-ships on the sea have their counterpart in the horrid little steamers that one now encounters high up the river. The number of these nuisances increases yearly at a greater rate than would be believed, and are fast robbing the river of its peaceful beauty. But have we not heard that even Venice, throned on her hundred isles, has had her hitherto silent thoroughfares invaded by one of these screeching little monsters ? The reflection most often forced upon our mind while engaged on the present work has been that, in whatever direction our study may have lain, "the old order changeth," and that had we delayed our task much longer there would have been left comparatively little of interest that an artist would select for representation. So, in the case of the locks themselves, the quaint old constructions of irregular wood- work that were a pleasure to look upon are gradually making way for successors of "improved" modern style. With side-walls of square blocks of concrete, and smooth gates as black as pitch can make them, they lose all charm of appearance. The action, too, of opening the gates by leaning the back against the swing-beam, that we have depicted, is fast becoming obsolete, giving way to a mechanical apparatus with wheel and axle.
Uses and effects of weir and lock (4): Locks as toll-gates
The locks also serve the purpose of toll-gates, the sum to be paid being regulated by the size or ft-eight of the boat passing. The proceeds are devoted to the necessary expenses connected with the navigation. There used to be considerable difference in the charges at the different locks under the old regime, some few of them, however, being free. At the present time all are under the management of the Thames Conservators, who have issued by-laws with the following scale of tolls for pleasure-boats. Class I.— For every pair-oared row-boat, skiffi outrigger, randan, dinghy, punt, canoe, or company-boat, id. Class II. — For every four-oared row-boat (other than the boats enumerated in Class I.), bd. Class III. — For every row-boat, shallop, and company-boat, over four oars, 9^1 For every house-boat, is. bd. The above charges to be for passing once through the lock, and returning the same day. In lieu of the above tolls, boats may be registered on the annual payment to the Conservators of the under-mentioned sums, and may, in consideration of such payment, pass the several locks free of any other charge : — Every rowboat in Classes I., II., and III. to pay respectively 20J., 30J., and 40J. per annum; and every house-boat, loos. per annum. . . .
The occupants of pleasure-boats frequently have a dread of passing through a lock, from an exaggerated idea of the danger of the proceeding; quite as often they are not aware of what danger there actually is; and hence many a day's pleasure has been marred. The safe position for a boat in a lock is to be parallel to and close by one of the side-walls or another boat. She should be held to the side with a boat-hook by the oarsman in the bow-seat when ascending the river^ and by the steerer when descending. When this rule is attended to, the pressure of the current itself keeps the boat in its proper position alongside, and prevents it swinging across the lock. The only case in which, to our knowledge, the above rule admits of any modification is when so strong a wind is blowing up the river as to counteract the pressure of the stream. In ascending, it is necessary to look sharply that neither a row-lock, nor any other part of the boat, gets caught under any projection, such as a beam, at the side of the lock, as in this way a boat will be first held by the rising water, then soon filled and swamped. Should, through carelessness, a boat become fixed in the way we are speaking of, the lockkeeper should be instantly shouted to, that he may let down the valves or paddles, and so prevent any more water coming in. While descending the  river, the danger is so slight that we have never known any case of an accident happening in a lock. If there should happen to be any gpreatly projecting ledge — a very rare occurrence — care must be taken that the boat do not rest at all upon it while the water is subsiding.
It is supposed, and with considerable probability, that the casual position of two weirs near each other may have originally suggested the invention of the lock. A number of locks on a river changes the naturally inclined plane of the water into a series of comparatively level surfaces, separated by abrupt descents; a somewhat parallel case on land would be to alter an easy slope into large flat terraces with a single step down from each successive terrace. Barge. [S5]
Robertson, H. R. Life on the Upper Thames. London: Virtue, Spalding, and Co., 1875. Internet Archive version of a copy in the Harvard College Library. Web. 29 April 2012.
Last modified 30 April 2012