80 
GAKDEN MECHANICS. 
ram at each movement or elevation of the water, from whence 
it is driven by the next movement into the air vessel, which would 
otherwise become filled with water under a high pressure. In the 
interior of the head of the ram is an annular space surrounding 
the frame of the stop valve, containing a small volume of air, 
which cannot be forced into the air vessel, but which at the first 
instant is compressed by, and receives the effort of, the moving 
water. This is called the air matrass, and by means of which 
the stop valve is shut with less noise, the pipe having a di¬ 
minished strain thrown upon it, and all the operations taking 
place with so much ease that the machine is less exposed to be 
shaken and put out of repair. 
“ The end of the pipe or body which receives the water of the 
reservoir is formed like a trumpet mouth, the length of it being 
regulated by the fall of w'ater which is to produce the current. 
The pipe is composed of several lengths, screwed together by 
flanches, or other similar means; but it is in the end piece, 
which is called the head of the ram, that the moving parts of the 
machine are placed. This extremity is a hollow sphere, the dia¬ 
meter of which is nearly twice as great as the bore of the pipe; 
the upper part of the spherical end is flattened, so as to reduce 
it to the segment of a sphere with a flat circular surface on the 
top, in the centre of which surface is a large circular opening to 
receive and hold the seat of the stop valves at which the water 
issues, but when the valve is closed it prevents the water from 
passing. By this arrangement, when the valve opens the water 
descends perpendicularly into the hollow sphere, leaving a free 
passage through the opening. The valve is guided between three 
or four perpendicular stems, which have hooks formed at the 
lower ends, by which it is supported when opened; these stems 
are fixed by screws, so that they can be regulated to allow the 
valve to descend more or less, and open a large or smaller passage. 
The valve is hollow, having a flat circular plate of metal, with a 
hollow cup or dish of the same material attached to its lower 
surface; this renders the valve lighter in the water, giving it a 
convex surface on the lower side, which, when opened, corre¬ 
sponds in curvature with the inferior concave surface of the spheri¬ 
cal end of the head of the ram. The seat of the valve is com¬ 
posed of a short cylinder or pipe, the opening of which is 
