1536

THEORY AND PRACTICE OF ENGINEERING.

Book II.

360 feet, the water drawn from the interval, for the purpose of mounting the boat, wouldlower it nearly 26 inches, arid there would not remain sufficient to keep it afloat; conse-quently, it would be necessary to draw a lockful from the upper interval, and then a secondto cause it to rise, whilst only one would be required if the locks were at a sufficient distance.

This example will show the inconvenience of having locks too near each other, which isstill further increased when they are contiguous: it frequently happens that several boatsarrive together in the same interval, particularly where the bargemen stop or sleep, and thatno water may be lost, the interval where they stop should be sufficiently long to admitmore than one; if circumstances will not permit this, a greater width must be given,that the lockfull which the rising boats draw from the interval should not cause the waterto lower so considerably as to prevent their floating, or the descending boats force insuch a quantity as to make it run over the gates: if the interval has only the ordinarywidth of 48 feet, it should be 6398 feet in length, so that ten rising boats could stop, ifnone were descending at the same time, otherwise a part of the water must be drawn fromthe other intervals to keep them afloat: if there were as many ascending as descendingboats, this need not be so great; but this observation proves that in forming a canal it isnecessary to have basins at those situations where boats are required to stop any length oftime.

Quantity of Water expended by Boats in traversing a Canal. — It was the opinion ofMM. Gabriel and Abeille, that the passage of a boat through the whole length of a canalalways cost twice the quantity of water necessary to fill a lock: Belidor thought the same,and it is still the common opinion. M. Thommason has nevertheless maintained that thisidea is erroneous, and that when one boat passes several locks one after another, the secondboat only expends two locksfull in its whole passage; but when they pass alternately, oneup and the other down, that it costs as many locksfull as there are locks in the ascensionof each boat. He founds this assertion on two statements, one of M. Caligny, the other ofM. Regemorte, asserting that the expenditure of the water is the same, whether contiguousor separated; but this distinction not having been sufficiently examined, a second error hasbeen committed; but it is undoubted that when locks are contiguous, they often expendmore than two locksfull; and it has not been remarked that when the locks are more than640 feet apart, they often expend only a single lockfull for the whole journey. Whenlocks are distant from each other, and the boats pass alternately, one up and the otherdown, the boat which passes after the first frequently finds in mounting all the locksempty, and to fill them it must draw a lockfull from each interval and one from thestarting point; in descending, as it finds the locks full, it does not draw any from the startingpoint, consequently it will only expend a single lockfull in its whole voyage.

When the locks are distant from each other, and the boats follow, the second boat willfind all the locks full going up, and to ascend it must first empty all, and then fill themwith water drawn from the intervals, and the highest from the starting point: in de-scending, all the locks will be empty, and the first lock will be filled with water from thestarting point, which will serve to fill all the others, so that this boat will expend twolocksfull in its journey.

When the locks are so near each other that the water of one taken into the intervalbetween the two diminishes the depth of this interval sufficiently to impede the navigation,or when the locks are contiguous and the boats pass alternately, the second boat inascending finds all the locks empty, and as it cannot draw water from the intermediateintervals from the contiguity of the locks, all are filled with water from the starting point.Thus in ascending each boat expends as many locksfull as there are contiguous chambers;in descending, all the locks being full, no water need be drawn from the starting point,consequently in a whole journey as many locksfull will be expended as there arecontiguous locks in ascending. When the locks are contiguous, and the boats pass eachother in succession, the second In ascending will find all the locks full, and to enable it toenter the intervals, it must empty them successively to fill them with the water from theintervals, except the last, which it fills with water from the starting point; in descending,another lockfull is taken from the starting point, so that in this case two locksfull aretaken from the latter.

Although the four above cases contain the whole theory of the working of locks, it maybe remarked that if two boats meet at the starting point, and two others before or afterthe starting point, the four will expend five locksfull; if two boats meet at the startingpoint, and the two following meet there also, the four will only expend four locksfull; if thetwo last boats that have passed meet before or after the starting point, and the two succeedingmeet also before or after the starting point, then they will only expend four locksfull,nad the first come in an opposite direction to that which had passed previously, and five ifit had come in the same direction ; and it has been generally observed, that a boat alwaystakes a lockfull from the starting point to ascend, but that it often does not take any todescend on the other side, consequently, when there are no contiguous locks, the boats willonly expend a lockfull for their whole journey, when they pass the starting point al-