'Transactions of the Bombay Geographical Society, from September 1850 to June 1852. Edited by the Secretary. Volume X.' [‎31] (52/458)

THE SOCIETY’S BAROMETERS. XXXI
generally been placed upon it. When instances were quoted of elevations determined by the
theodolite and the barometer agreeing to within a few feet of each other, as illustrations of the
length to which accuracy might be carried, there might in reality be very little merit due
to any one -the coincidence being matter of accident, and in not one experiment out of
ten would the same results be attained by the same observer, making use of the same instruments
under circumstances as nearly as possible identical. The error might not be very great, but it
was well to know that it was impossible to provide against the elimination of error altogether.
The experiment bore on another point: in the examination of that very ill understood phenome
non, the daily, fluctuation of the barometer, it was considered matter of much importance
to know at what rate the amount of tide decreased as we assended from the surface of the
earth. The best experiments on this subject were those made simultaneous by Colonel Sykes at
Poona and Dr Walker at Mahabuleshwar—the difference of level being 2500 feet; with those
made at Madras, twenty feet, Trevandrum, 150 feet, and Dodabetta, 8500 feet, above the level
of the sea, and those made at Aden by Mr Mayes on the Seerah Island, 180 feet, and on
the top of Shum Shum, 1700 feet above the level of the sea. In this last case the tide at
the higher point ought to be about one twentieth less than that of the lower: at Dodabetta,
it ought to be one third less than at Madras, and less they were accordingly, in somewhat like
these proportions; but until either better instruments than the best of our barometers were
procured, or a much larger mass of observations than that now existing were accumulated,
it would be unsafe positively to lay down the law on the subject. No sufficient explanation
of these discrepancies had been given : Dr Ford ascribed them to the electric state of the baro
metric tube, to which very probably might be due. In want of explanation of their cause, it was 5
well to be in possession of the fact of their existence. (See next page.}
The Aneroid is an instrument of so much value for survey purposes in India, that I hav e
lost no opportunity of endeavouring to ascertain its trustworthiness, and in general the
results of these inquiries have redounded greatly to its* praise. An Aneroid appears to have
been kept at the Colabah Observatory for the past twelve months, comparative readings of which
with the Standard Barometer have been published in the Newspapers.
The following is an abstract of these for the while of the year 185(1. From this it will
be seen that the range of the Aneroid which at the commencement of the year was close on that
of the Barometer has towards conclusion been reduced to very nearly a fourth of that instru
ment. The standard Barometers at the Observatory are the finest instruments that can be made,,
and there seems no reason to doubt the perfect care and accuracy with which they ar*
observed. Of the Aneroid in use I cannot speak, but it is observed, I presume by the same
parties who read the Barometer, and has therefore ikely to be treated with every justice.
If under these circumstances the Aneroid is apt to deteriorate so fearfully, our faith m
it must he materially modified, and those instruments only trusted which have before hand hem
mbj ected to long and careful examination, (See page xxxiii)j.