'The Transactions of the Bombay Geographical Society. From January 1863 to December 1864. (Edited by the Secretary.) Volume XVII.' [‎6] (167/524)

6
REMARKS ON THE
earth ; but he does not state how he derived them ; they agree very
closely, however, with those derived by Laplace in his earlier Memoir
on the Brest tides, from which he obtained as the moon’s mass, and
18° as the elongation at the time of mean retardation of the tide at
Brest, whilst he made the moon s effect in raising the tide 2'6l6/
times the sun’s. Now, as these are the quantities from which the
principal correction or <f semi-menstrual inequality” is obtained, iti s
of importance that they should be determined with all practicable
accuracy. From want of time, I have been unable to verify the
above results by an investigation of the values given by the heights
of the tides. That they are fair approximations to the true values,
however, may be inferred from the confirmation they obtain from
other sources. Thus, in the investigation by the Astronomer Royal
respecting the tides in certain Irish ports, he found the values of the
ratio I- at Kilbaha ■= *41, Casleh Bay ‘43, Galway -43, Old Head *42,
Mullaghmore *40; the mean of which values is *418, and its reciprocal,
or the ratio of the moon’s force to the sun’s 2*39, which makes the moon’s
mass or of the earth’s. Moreover, though in his first memoir
Laplace found the moon’s mass from the Brest observations ^ of the
earth’s, in a later discussion, of a more extensive series of observations
at the same place, he found it Again, from purely astronomical
data, Baily gives the same quantity ^77, Peters and Schlidioffsky gE,
and Adams It may be inferred then, that the value - 7( |-, as em
ployed by Professor Orlebar is probably too large, and the dependent
ratio re( l u i res 1° k 6 proportionately altered.
The effect of changing the mean establishment and moon’s mass
to the extent just indicated, would be to increase the semi-menstrual
inequality, and make the predicted time of high-water later by varying
amounts. This seems to be required ; for, returning to the observations,
it is found that, by dividing the algebraic sum of the difference*
between the computed and observed times of high-water by the number
of observations in each case, the mean difference at upper transit
is and at the lower transit the mean difference of the
day tides only at the upper transit i0‘8?w. and at the lower 3 ' 3 wn, and
for the night tides 14 m. and 10m. respectively; and, finally, of the
whole 9J minutes, all these quantities being affected by the same sign,
and indicating that the mean computed times of tide are too early.