282

RIVER AND INLAND NAVIGATION.

Parts of locks.

Form.

Fig. 22.

Locks on the Crozat Canal.

42. The parts of a lock may beconsidered^ as being, firstly, theupper chamber, or head, with itsgates, and the breast-wall; 2dly,the intermediate lock - chamber,between the breast-wall and theextreme point of the projectionof the lower gates; and 3rdly,of the lower chamber.

43. On many canals the formof the lock-chambers is madesuch as to give the side-walls

LONGITUDINAL

SECTI

a concave direction towards the water, with the intention of opposing a greaterresistance to the pressure of the earth. There does not appear to be anythingreally gained by this course, because, even supposing the walls do resist the thrustof the embankment more effectually in this shape with the same quantity ofmasonry, locks so constructed waste more water, and moreover the extra thicknessrequired to be given to the floor on account of the increased span nearly equals thesaving in the walls. The best modern canals are made with locks of a rectilinearform, similar to those indicated in the above sketches. Sometimes the sides havebeen executed in slope, but the loss of water at every passage is enormous, and theprolongation of the wing-walls of both the upper and the lower chambers is oftenmore than enough to compensate for the small saving of the side-walls, especially

Fig. 23.

SECTION ON A.B

SECTION ON CO,

21 4 '

mmm.

SiitliiSIH

Fig. 24.