co 
ON WAVES. 437 
directly observed in the glass indices, figure 6, on the scale of 
which the deviation from approximate depth, already given at 
the head, (Depth, 4 inches,) is read off with the appropriate 
sign + or —; and the mean depth of the fluid having been al- 
ready compared by direct experiment with the scale of the 
index, and a correction for error of scale applied, the true result 
is given at the end as the mean depth of the fluid when at rest, 
freed from instrumental error, thus : 
y = — 0°05 
Statical level observed at { 5 — _ 0-01 
\ corrected statical depth=3-942 inches. 
In the table of the observations, column A gives the number 
of feet passed over by the wave, reckoning from the instant at 
which the first observation of time in codwmn B was made on 
either or both of the chronometers a and 8. In column C are 
given the readings of the index y at that end of the reservoir 
where the wave was generated, and from which the observations 
are begun, and of the index 6 placed towards the other end of 
the reservoir. In colwmn D the observations of column C have 
been freed from the error of the index scale, so as to represent 
the true height of the ridge of the wave above the statical level 
of the fluid; and in column E the true height of the wave has 
been added to the statical depth of the fluid, so as to give the 
whole depth reckoned from the ridge of the wave to the bottom 
of the reservoir. 
The observations were made in the following manner. The 
wave having been generated, was generally allowed to traverse 
the whole length of the reservoir, and return to y before com- 
mencing the observations of time and space; this was done for 
the purpose of allowing the wave to assume its determinate 
form, which it did not generally acquire until it had remained 
for some time unaffected by external impulse; and this delay 
also allowed the secondary oscillations of the fluid to disappear. 
On the return of the wave to y its height was carefully ob- 
served ; after passing y its transit past the central station was 
assumed as the zero for time, its height was observed at 6, and 
once more on its return to y, so that. the interval between the 
observations was an interval due to 20 feet or 40 feet, accord- 
ing as the observations were made on successive or alternate 
transits ; the successive transits being used when the velocity 
was small, and the alternate ones when the velocity was such 
as not to afford sufficient intervals for observing and noting with 
composure. 
The intervals between the transits were obtained with con- 
siderable precision, as may be gathered from the following 
