ON WAVES. 433 
bability in favour of accuracy in the result is elevated to the 
region of certainty. It was found that when a smooth plane sur- 
face, of sufficient rigidity, was immoveably fixed at the end of 
the channel, at right angles to the direction of the wave’s trans- 
mission, the wave was thereby reflected without sensible change 
in its form, magnitude, or velocity. Two such reflecting sur- 
faces being placed at opposite ends of the reservoir, it was found 
that the wave might be reflected from one end to the other over 
successive spaces of 20 feet, and thus brought repeatedly to the 
same points of observation. In this way the same wave was 
observed during so many as 60 successive transits after 60 suc- 
cessive reflections, having thus passed over a course equal in 
length to 1200 feet, and occupying an interval of 320 seconds, 
giving the power of observing it 60.times in its transit past a 
given point. It was thus brought under the eye of three ob- 
servers at three different parts of the reservoir during a single 
transit. The whole internal surface of the reservoir was accu- 
rately divided into feet, inches, and minuter divisions. 
Means of observing the transit.—To observe the instant of 
the transit of a wave past a given point is a matter of some 
difficulty, especially when the wave is long and flat. A wave 
one-tenth of an inch high and three feet long is scarcely sen- 
sible to the eye until its vertex has passed ; its commencement 
and end are perfectly insensible, and its summit so flat that it 
is impossible directly to observe its place with precision. To 
obviate these difficulties, the following apparatus was provided. 
A plane mirror, M, (Fig. 2. Plate 1.) was raised on a frame to a 
height of four feet above the surface of the water. On this 
mirror the image, I, of a bright flame was thrown, and the mir- 
ror was adjusted so as to reflect this image upon the surface of 
the water (at W). A second mirror (m) was placed over this 
second image, so as to intercept the rays reflected from the sur- 
face of the water, and to return them finally through an eye- 
piece to the observer. The path of the ray was preserved 
during the whole of its extent in a plane at right angles to the 
direction of the motion of the wave. Parallax in observation 
was avoided by a micrometer wire in the eye-piece, which was 
kept in coincidence with an opaque line passed through the 
image at M and so reflected in m, and with a line of division, D, 
seen directly without reflection past the edge of the mirror m. 
The observer was thus enabled to compare the place of the 
centre of the reflected image by coincidence with fixed lines. 
When perfectly at rest the coincidence was perfect. When the 
centre of the wave was at W"", figs. 2 and 3, the rays of light also 
reflected from a plane surface, perfectly horizontal, presented the 
VOL. VI. 1837. QF 
