1538

THEORY AND PRACTICE OF ENGINEERING.

Book II.

chambers of locks have a thickness relative only to the thrust of the earth, they may giveway when the earth is put in motion, which often occurs from a slight filtration behind thewall. Gauthey has a rule for finding the thickness to be given to the wall of a basin in-tended to support water throughout its whole height, and in the chambers of locks it mustbe remembered that the thrust of the water against the vertical surface is equal to the productof these surfaces by half the height of the water: call A the height of the wall, x =its thickness,supposing its length to be 1 metre, the acting power will be 1000 x ^ A 8 : supposing thecube metre of water to weigh 1000 kilogrammes, and the centre of impression of this thrust

Fig. 2833. LONGITUDINAL SECTION.

Fig. 2833.

!i

Fig. 2834.

PLAN OF LOCK,

being at a third of the height of the wall, the arm of the lever of the acting power will beequal to \ A.

The resisting power will be the wall itself — Kx x 2000, supposing that the cube metreof masonry generally weighs 2000 kilogrammes. The arm of the lever will be half thethickness of the wall x, consequently the momentum of the acting power will be 1000x£A 2 x£A, and that of the resisting power 2000 x \hx *; and as in the state of equilibriumthese two powers should be equal, we shall have 167A 3 — 1000A# 2 , from whence we havex = A 3 = 0*41 A : but, as something should always be allowed above the equilibrium,

by adding we shall have x— |A nearly : hence it is evident that the thickness of a wallintended to support water should be equal to half the height of the water which actsagainst it.

The length and width of chambers of locks must necessarily be regulated in conformitywith the boats used on the canal; these are generally longer and narrower than those onrivers, where the shallows which occasionally occur require flatter bottoms to be giventhem: the letters AB, CD, EF, refer to the sections through the several parts of thelock chambers. With regard to the length of the chambers, it should be such as to enablethe gates at the lowest ends to open and shut easily; if the rudder of the boat cannot beunshipped, or occupies any portion of the length of the chamber, then the chambers mustbe made sufficiently long to prevent them from interfering with the opening of the gate, onwhich account the most proper rudders for navigable canals are those like broad oars,which can be taken out whilst passing through the locks. The height of the water in theintervals is regulated by the mean height of the waters of the river which communicatewith the canals : it is, however, customary to allow the latter a sufficient height of waterto receive boats of the same tonnage as those which navigate the river ; another advantage

Fig. 2835.

Fig. 2836

■TRANSVERSE SECTIONS.