434 SEVENTH REPORT—1837. 
same coincidence ; but when the anterior part of the wave W®, 
figs. 2and 3, was that on which the rays fell, the image was carried 
in the direction of the motion; and, on the other hand, when 
the posterior surface of the wave reflected the image, it was 
transferred to the other side, as in the point W™. When, 
therefore, the transit of a wave took place, the following phe- 
nomena presented themselves to the observer. The image con- 
tinued at rest, as seen in fig. 3, until the approach of the wave ; 
from the instant at which the transit began until the instant of 
the passage of the crest of the wave, the image appeared on the 
anterior side of the wire, as in fig. 4; but during the remainder 
of the transit, the image was found on the posterior side of the 
wire, as in fig. 5; and therefore the instant of the transit of 
the crest of the wave across the line was also the instant of the 
passage of the image from one side to the other across the wire: 
now, as the whole time of the transit did not amount to a second, 
this instant was given with the required precision, and although 
the elevation of the surface was not in many cases perceptible to 
the eye, the transit of the image was perfectly satisfactory. 
For obtaining the dimensions of the wave with precision, 
various expedients were resorted to ; there were provided glass 
tubes (gauges or indices) communicating with the channel at 
different depths ; they are represented in fig. 6. The centre of 
each tube opens into the side of the reservoir at successive 
inches of its height, and after continuing horizontally for a cer- 
tain space, is turned up vertically, and rises above the level of 
the water; the tubes thus become filled, and the water in each 
tube being tinged with colouring matter becomes distinctly 
visible, so that the variations of height are read with ease and 
precision on the graduated scale behind the tubes to hundredths 
of an inch. For a very elegant method of ascertaining the 
length of the wave with precision, Mr. Russell is indebted to 
Professor StEVELLY of Belfast, who suggested that fine points, 
similar to those used in the standard cistern barometers, should 
be applied to the surface of the water, so as to show by the 
instant of their submersion in the fluid, or emergence from it, 
the origin and end of the wave. This method was found to 
possess much precision ; the phenomena of capillary attraction 
mark the instants of contact and separation with vividness, by 
the reflection of rays of light from the concave surface of the 
fluid raised around the point, and their disappearance on sepa- 
ration. The contact of this point with its image in the water 
was also a phenomenon marking the place of the surface of the 
fluid with minute accuracy. When the two points, placed at 
the beginning and end of the wave, showed the phenomena of 
