'The Transactions of the Bombay Geographical Society. From January 1863 to December 1864. (Edited by the Secretary.) Volume XVII.' [‎10] (171/524)

10
REMARKS ON THE
in which A and 13 are found for each hour from the observations
In attempting to solve so large a group of equations, however, irregu
lari ties became evident, and showed that the observations must
first be examined individually by some approximate method. N 0w
Captain Barker informed the Society at the last meeting, that by
adding Vh. 10 m. to the time of the preceding tide at London Bridge
the local time of high-water at the Bombay dock gate was ob
tained with considerable accuracy. This supposes the age of the tide
at Bombay above days. The Government Observer also stated
that he was in the habit of subtracting 2A. 50m. from the date
of the London tide for that of the Bombay tide, and preferred
this method as more accurate than Professor Orlebar’s formula,
though the Observatory assistants were allowed to use that formula
in the computations supplied to the Bombay Almanacs. This method
supposes the age of the tide to be fully 2 days; —the constant being
the sum of the establishment of the port at London, which is 2A. 7m.
after the moon’s transit at new and full moon, the difference of the
times of transit, which is about 10m. earlier at Bombay • than at
London, and the establishment at Bombay, which—as shown in my
former paper—is about 33m. before the time of transit. Then the
interval between two successive tides at the times of conjunction and
opposition being about \2h. 21m., if we subtract 2A. 50m. from this
interval, we obtain 9A 31m. —this constant, which for convenience of
reference I shall call the first, or Captain Barker’s, .ought to be added
to the times of the London tides to give the tide date at the Colaba
Observatory. The observed times of high-water at the syzygies, I have
remarked, are very accordant, and the mean times of all the tides at
new and full moon, taken from the observations for 1861, and compared
with the corresponding London tides, confirms the values of 9^. 31w.
and 2A. 50m. obtained for these first and second constants.
Here, then, are two methods of testing the accuracy of the observa
tions, suggested by gentlemen who have opportunity of knowing their
accuracy, and who have declared them to be good approximations.
Theoretical considerations, however, lead us to suggest:—First, that ot
all ports, London is among those worst situated as a port of reference,
being near the mouth of a considerable river falling into a sea that
receives two tides differing by half a tidal day in age, and whose
compound effect is the London tide. Brest, so celebrated in connection