ON WAVES. 435 
immersion and emergence at the same instant, their distance 
was equal to the length of the wave. It was, however, neces- 
_sary to have some means of bringing both points under the eye 
at the same instant, in order to determine with accuracy the 
coincidence of contact in both cases; the arrangements are 
given in fig. 7. P and P are points in contact with the surface 
of the fluid at the extremities of a wave; rays of light from 
them are reflected by the mirrors p and p to the eye at O, and 
are thus observed simultaneously. By these means, the points 
being removed further apart, or brought nearer, until the con- 
tact became simultaneous, and the distance of the points equal 
to the length of the waves, the height of the wave was de- 
termined by the glass indices in fig. 6. 
Apparatus for generating the Waves.—Generating reservoir 
A. fig. 8, consisted of a continuation of the experimental reser- 
voir A, B, D, of fig. 1, which was separated from it or con- 
nected with it by means of a sluice; so that by filling the 
generating reservoir with water to a higher level than the ex- 
perimental reservoir while the sluice was closed, on raising it 
the water descended, producing a wave, of which the volume 
was known. The area of the horizontal section of the generating 
reservoir is 76°27 square inches, its length being 6°33 inches in 
the direction of the motion of the wave, and 12°05 inches its 
breadth at right angles to this ; the detached generating cham- 
ber B, fig. 9, was a rectangular parallelopipedon, open at top and 
bottom, and so accurately fitted to the bottom of the reservoir 
as, when resting on it, to be capable of containing water to any 
height, but on raising it from the bottom by which it had been 
thus temporarily closed, the fluid descended, producing a wave 
of given volume. The area of the horizontal section of the 
chamber is 68°32 inches, being 6°1 inches long and 11°2 inches 
wide. A solid parallelopipedon, C. fig. 10, was used to generate 
waves, by protruding it to a given depth in the fluid; the area 
of its horizontal section being 88°32 inches, and its dimensions 
24:0, 12°05, and 7°33 inches. Another detached generating 
chamber, D., was 2°98 inches, being 11°92 inches broad and 
24° inches deep, being an area of 35°52 square inches in its ho- 
rizontal section. In those cases where volume of the wave was 
not of importance, the wave was produced by the impulse of a 
flat surface pressed horizontally on the fluid. 
Analysis of Experiments.—The original experiments are 
themselves given at the end of this paper, for the purpose of 
enabling any one who may be disposed to make use of them 
for any future purpose, either of framing or testing a theory, to 
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