Locks are barriers or doors constructed so that these artificial rivers may be carried over rising ground and through valleys, without the labour and expense of cutting through the hills and filling up the hollows as would be required without them. Railways can be constructed on ground which is not quite level without any embankment or cutting, as it is not absolutely required that railways shall be perfectly level. But water always will be level, unless it is constantly flowing, as is the case with streams, and these only sink a few inches in a mile, or else they become so rapid, that if attempted to be imitated in canals, they would be useless; for it would require too much power to draw any vessel up the canal against such a stream, and would moreover require more water to supply them than can commonly be obtained. The means therefore adopted to overcome this difficulty are gates, or in other words, a pair of “locks.” The canal is constructed in such a manner that it shall be perfectly level for a certain distance, then sink down some ten or twelve feet at once, and again flow on a level and sink down. These sudden lowerings are effected as follows: two pairs of thick solid doors of wood are fitted to shut in the water, and another pair a short distance further on; behind these the bed of the canal is lowered the required distance. When a barge or other vessel has to pass down the canal the first pair of gates are opened, and the barge floated in between them and the second pair, the first pair are now closed, and the water beyond the second pair being lower, that between the gates in which the barge floats is let out by means of a valve worked by a rack and wheel; when this valve is raised the water flows out and sinks down to the level of the water beyond, carrying the barge with it; in a few minutes it is so low that, the second gates being opened, the barge is drawn out and continues on its way. But suppose the barge had to be brought up the canal, then it is floated into the space between the gates (as before), and the one behind the barge closed, the water beyond the gate in front being higher, is let into the space where the barge is by a valve, and this filling, lifts up the barge to the level of that in front, the front gates are then opened and the barge proceeds onwards. These gates are never made to shut level, but meet at an angle with the point towards the highest water, which is done that they may resist the great pressure which the water exerts, and for turning this to advantage, for this very pressure shuts the gates and keeps them close together. The canals which have much traffic on them have double locks, as in the engraving, that barges may go up and down at the same time without having to wait for each other. The barges and boats are generally “towed” or drawn by a horse attached to a rope, and walking on a “towing path” or road at the side of the canal. THE END.