The amplitude is greatest (0·088) in the spring months (March, April, and May), both at Dorjiling and Calcutta: it is least at both in June and July, (0·027 at Dorjiling), and rises again in autumn (to ·082 in September).
The horary oscillations also are as remarkably uniform at all elevations, as the period of ebb and flow: the mercury falls slowly from 9.50 a.m. (when it is at its highest) till noon, then rapidly till 3 p.m., and slowly again till 4 p.m.; after which there is little change until sunset; it rises rapidly between 7 and 9 p.m., and a little more till 10 p.m.; thence till 4 a.m. the fall is inconsiderable, and the great rise occurs between 7 and 9 a.m.
It is well known that these fluctuations of the barometer are due to the expansion and contraction by heat and moisture of the column of atmosphere that presses on the mercury, in the cistern of the instrument: were the air dry, the effect would be a single rise and fall;[[420]] the barometer would stand highest at the hottest of the twenty-four hours, and lowest at the coldest; and such is the case in arid continental regions which are perennially dry. That such would also be the case at Calcutta and throughout the Himalaya of Sikkim, is theoretically self-evident, and proved by my horary observations taken during the rainy months of 1848. An inspection of these at the end of this section (where a column contains the pressure of dry air) shows but one maximum of pressure, which occurs at the coldest time of the twenty-four hours (early in the morning), and one minimum in the afternoon. In the table of mean temperatures of the months, also appended to this section, will also be found a column allowing the pressure of dry air, whence it will be seen that there is but one maximum of the pressure of dry air, occurring at the coldest season in December, and one minimum, in July. The effect of the vapour is the same on the annual as upon the diurnal march of the pressure, producing a double maximum and minimum in the year in one case, and in the twenty-four hours in the other.
[420] This law, for which we are indebted to Professor Dove, has been clearly explained by Colonel Sabine in the appendix to his translation of Humboldt’s “Cosmos,” vol. i. p. 457.
I append a meteorological register of the separate months, but at the same time must remind the reader that it does not pretend to strict accuracy. It is founded upon observations made at Dorjiling by Dr. Chapman in the year 1837, for pressure temperature and wet-bulb only; the other data and some modifications of the above are supplied from observations of my own. Those for terrestrial and nocturnal radiation are accurate as far as they go, that is to say, they are absolute temperatures taken by myself, which may, I believe, be recorded in any year, but much higher are no doubt often to be obtained. The dew-points and saturations are generally calculated from the mean of two day observations (10 a.m. and 4 p.m.) of the wet-bulb thermometer, together with the minimum, or are taken from observations of Daniell’s hygrometer; and as I find the mean of the temperature of 10 a.m., 4 p.m., and the minimum, to coincide within a few tenths with the mean temperature of the whole day, I assume that the mean of the wet-bulb observations of the same hours will give a near approach to that of the twenty-four hours. The climate of Dorjiling station has been in some degree altered by extensive clearances of forest, which render it more variable, more exposed to night frosts and strong sun-heat, and to drought, the drying up of small streams being one direct consequence. My own observations were taken at Mr. Hodgson’s house, elevated 7,430 feet, the position of which I have indicated at the commencement of this section, where the differences of climate due to local causes are sufficiently indicated to show that in no two spots could similar meteorological results be obtained. At Mr. Hodgson’s, for instance, the uniformity of temperature and humidity is infinitely more remarkable than at Dr. Chapman’s, possibly from my guarding more effectually against radiation, and from the greater forests about Mr. Hodgson’s house. I have not, however, ventured to interfere with the temperature columns on this account.
DORJILING METEOROLOGICAL REGISTER.
| Jan. | Feb. | Mar. | Apr. | May | June | |
| Pressure of Atmosphere[[421]] Range of Pressure Mean Shade Maximum Shade Maximum Sun Greatest Difference Mean Maximum Shade Minimum Shade Minimum Radiation Greatest Difference Mean Minimum Shade Mean Daily Range of Temps Sunk Thermometer Mean Dew-point Mean Dryness Force of Vapour Pressure of Dry Air Mean Saturation Rain in inches | 23·307 ·072 40·0 56·0 119·0 72·0 47·2 29·0 16·0 12·7 32·8 14·4 46·0 34·3 5·1 ·216 23·091 ·84 1·72 | 23·305 ·061 42·1 57·0 124·0 78·0 50·0 25·5 23·0 15·3 34·2 15·8 48·0 37·2 3·9 ·239 23·066 ·87 0·92 | 23·307 ·083 50·7 66·5 120·0 60·0 58·4 37·0 27·8 8·7 43·1 15·3 50·0 45·8 5·8 ·323 23·084 ·82 1·12 | 23·280 ·085 55·9 68·5 125·0 66·0 63·7 38·0 33·0 16·0 48·1 15·6 58·0 49·8 6·6 ·371 22·909 ·80 2·52 | 23·259 ·088 57·6 69·0 125·0 65·0 65·3 38·0 40·0 10·0 50·0 15·3 61·0 54·4 2·7 ·434 22·825 ·91 9·25 | 23·207 ·067 61·2 71·0 126·2 62·2 66·7 51·5 47·0 4·8 55·8 10·9 62·0 59·5 2·0 ·515 22·692 ·93 26·96 |
| July | Aug. | Sept. | Oct. | Nov. | Dec. | Mean | |
| Pressure of Atmosphere[[421]] Range of Pressure Mean Shade Maximum Shade Maximum Sun Greatest Difference Mean Maximum Shade Minimum Shade Minimum Radiation Greatest Difference Mean Minimum Shade Mean Daily Range of Temps Sunk Thermometer Mean Dew-point Mean Dryness Force of Vapour Pressure of Dry Air Mean Saturation Rain in inches | 23·203 ·062 61·4 69·5 130·0 62·0 65·5 56·0 52·0 3·5 57·3 8·2 62·2 60·7 0·8 ·535 22·668 ·97 25·34 | 23·230 ·070 61·7 70·0 133·0 62·0 66·1 54·5 50·0 3·5 57·4 8·7 62·0 60·4 1·1 ·530 22·700 ·96 29·45 | 23·300 ·082 59·9 70·0 142·0 70·0 64·7 51·5 47·5 10·0 55·2 9·5 61·0 58·5 1·4 ·498 22·802 ·95 15·76 | 23·372 ·075 58·0 68·0 133·0 65·0 66·5 43·5 32·0 12·0 49·5 17·0 60·0 52·5 4·2 ·407 22·865 ·86 8·66 | 23·330 ·078 50·0 63·0 123·0 68·0 56·5 38·0 30·0 12·0 43·5 13·0 55·0 46·5 3·2 ·331 22·999 ·90 0·11 | 23·365 ·062 43·0 56·0 108·0 77·2 51·6 32·5 26·0 10·0 34·9 16·7 49·0 31·8 10·6 ·198 23·165 ·69 0·45 | 22·289 ·074 53·5 65·4 125·7 67·3 60·2 41·3 35·4 9·9 46·8 13·4 56·2 49·4 4·0 ·383 22·906 ·88 Sum 122·26 |
[421] These are taken from Dr. Chapman’s Table; and present a greater annual range (=0·169) than my observations in 1848–9, taken at Mr. Hodgson’s which is higher than Dr. Chapman’s; or than Mr. Muller’s, which is a little lower, and very near it.
Horary Observations at Jillapahar, Dorjiling, Alt. 7,430 feet.
JULY, 1848