MEANS OF COMMUNICATION. 51 
reservoir of St. Ferreol, of which pl. 11, fig. 1, is a section, and jig. 2 a part 
of the ground plan. The inner or principal wall of this reservoir is 110 
feet high, and contains about sixteen hundred thousand cubic feet of 
masonry. To about 40 feet from the bottom this wall is 40 feet in thick- 
ness ; then suddenly contracting to 18 feet, it tapers to the top, where it is 
about six feet thick. At a distance of 200 feet from the inner wall, the 
outer wall is built 64 feet high, and the space between the two walls is filled 
with binding clay, which is well rammed. Through an arched aqueduct 
which commences about the middle of the reservoir (D, jigs. 1 and 2) the 
water is let into the feeder-canal through guard-gates, at a rate insuring 
safety to the works. The reservoir is 530 feet long and 265 feet wide. 
Another important reservoir of this canal is that at Lampy: pl. 11, jig. 3, 
represents a side elevation of it; jig. 4, a section through a, and fig. 5, a 
section through B. The water can be let off at different heights, as seen at 
A and B. 
C. Locks. 
A lock is a small basin which connects different levels of a canal, and 
through which boats ascend or descend from one level to the other. The 
bottom of the lock is even with that of the lower level, and the top is even 
with that of the upper level of the canal. Both ends are closed by gates 
provided with valves, through which the water can be let into the lock from 
the upper level, or lowered to the height of the lower level, the lock form- 
ing thus at pleasure a continuation of either portion of the canal. When a 
boat is to descend through the lock, the water is let in from above, the upper 
gates are opened, and the boat is drawn into the lock; the upper gates are 
then closed, and the valve in the lower gate is opened, when the boat will 
descend with the water to the lower level, and on the lower gates being 
opened it can continue its course. The inverse operation will be pursued 
when the boat is to ascend to the higher level. 
From this it is apparent that locks form the most important feature in 
the construction of a canal. By means of them navigation is made practi- 
cable and easy where otherwise it would be impossible. Their use is not 
confined to canals proper, but they are also used where shallow streams 
have been made navigable by means of damming up successive portions, 
-as mentioned in the introduction to this subject. 
The construction of locks demands the greatest attention on the part of 
the engineer, as the pressure which they sustain and the action of the water 
upon them are greater than at any other point in the canal. The size and 
proportions of a lock are dependent upon the size of the boats to be used, 
the frequency of navigation, and the supply of water. The form is usually 
rectangular, unless a special object dictates a different form; pl. 11, fig. 12, 
is a top view of a lock, which stands by the side of a wooden dam in one 
of the canals in Russia; jigs. 13 and 14 are the two side views, and jig. 15 
asection. Pl. 10, figs. 6, 18, and 21, give the top views of different locks 
in the Languedoc Canal in France; jigs. 2 and 4 show cross-sections, and 
Jigs. 19 and 20 longitudinal sections of various other locks, and jig. 29 is a 
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