XV. Discussion of meteorological observations taken in India, at various heights, embracing those at Dodabetta on the Neelgherry Mountains, at 8640 feet above the level of the sea

Abstract

In the year 1835 the Royal Society did me the honour to publish the results of six years’ meteorological observations, taken by me on the elevated plateau of the Dukhun (Deccan) from 1825 to 1830, both years inclusive; my chief object being to illustrate the diurnal oscillations of the barometer as indicative of the periodic tides (if they may be so called) of the atmosphere. I showed that in many thousand observations, taken personally, there was not a solitary instance in which the barometer was not higher at 9—10 a.m. than at sunrise; and lower at 4—5 p.m. than at 9—10 a.m., whatever the state of the weather or the indications of the thermometer or hygrometer might be ; nor was there a solitary instance in 1830, during which year only I took systematic night observations, and observed at the turning-points every five minutes, in which the maximum nocturnal tide was not higher at 9—10 p.m. than at 4—5 p.m. The 4—5 a.m. minimum tide was less regularly noticed than the other three tides; but nevertheless sufficiently often to render it perfectly clear, that in the twenty-four hours there were two minima as well as two maxima of pressure in twenty-four hours. I had the advantage at times of carrying on my observations in the six years, at a mean elevation of 1800 feet, simultaneously with observations made in Bombay at the level of the sea, and at Mahabuleshwur at 4500 feet above the sea. These simultaneous observations for limited periods led me to remark, that the amount or range of the diurnal oscillation between the maximum and minimum hours did not correspond at the different elevations. At the level of the sea in Bombay the range between 9—10 a.m. and 4—5 p.m., appeared generally to be less than the range between the same hours in Dukhun (Deccan) at 1800 feet above the sea; but at Mahabuleshwur, at 4500 feet, the range was constantly less than in Bombay or on the plateau of the Deccan. Not having had further means of prosecuting inquiry into the fact, it was with much interest I remarked that the orders of the Court of Directors of the East India Company had been carried out by the astronomer at Madras, and meteorological observations taken for a whole year at a greater elevation above the sea-level, than had ever been attempted before in India. Observations so made were peculiarly acceptable to me, as they would subject the accuracy of my own observations to an additional test, and would supply the means of investigating the supposed diminished diurnal oscillation of the barometer in relation to elevation above the sea. At the present time I have the further advantage of being enabled to subject my Deccan observations to the test of comparison with recent observations at the Bombay and Madras Observatories, taken chiefly with a view to determine the amount of the hourly and diurnal changes of the barometer. Those for Madras are from the years 1841 to 1845, both inclusive: for Bombay, I am sorry to say, the records which have reached my hands are for more limited periods. Dr. Buist, LL. D., while in temporary charge of the observatory, made observations for 1842, 1843 and 1844, but those for 1843 and 1844 only are available to me, and Professor Orlebar has published observations from April to December 1845. In addition to the above, a meteorological register is kept in the office of the Deputy Surveyor-General in Calcutta, the instruments since 1844 being new, and of Newman’s construction; but hourly observations do not appear to have been taken, and three out of the five periods of observation, namely, sunrise, noon and sunset, are useless for determining the diurnal oscillation of the barometer; and the barometrical observations taken before the receipt of the new instruments, which came to hand respectively in 1844 and 1847, were recorded from defective instruments.