117. =Canals.=--The management of the locks of a canal is in conformity with the disposition of water to seek a level. A ground view of a lock and a part of two adjacent locks is given in Fig. 68. The lock, C, has two pair of flood-gates, D D and E E. The water in A is higher than in C, but the level is the same in C and B, because the gates, E E, are open. Suppose now that there is a boat in the lock B that you wish to get into the lock A. It must be floated into the lock C, and the gates E E must be closed. The water may now be made to flow from the higher level, A, into C, till the level is the same in both A and C. But this can not be done by opening the gates D D, for the pressure of such a height of water in the lock A would make it difficult, perhaps impossible, to do this; and besides, if it could be done, the rapid rush of water into C would flood the boat lying there. The discharge is therefore effected by openings in the lower part of the gates D D. These openings are covered by sliding shutters, which are raised by racks and pinions, as represented in Fig. 69. When the water has become of the same level in A and C, the gates DD can be easily opened, and the boat may be floated from C into A. If a boat is to pass downward in the locks, the process described must be reversed. Canals are also extensively used for supplying water by side openings to turn water-wheels for the working of machinery. The water turns the wheel by the force which gravitation gives it as it descends from the level of the canal to the level of the river. [Illustration: Fig. 70.] [Illustration: Fig. 71.]