That the Calcutta amplitude is not exceptionally great, is shewn by the register kept at different places in the Gangetic valley and plains of India, between Saharunpore and the Bay of Bengal. I have seen apparently trustworthy records of seven* [Calcutta, Berampore, Benares, Nagpore, Moozufferpore, Delhi, and Saharunpore.] such, and find that in all it amounts to between 0.084 and 0.120 inch, the mean of the whole being 0.101 of an inch.

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;* [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.] 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.

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* 23.307 .305 .307 .280 .259 .207
Range of Pressure .072 .061 .083 .085 .088 .067
Mean Shade 40.0 42.1 50.7 55.9 57.6 61.2
Max. Shade 56.0 57.0 66.5 68.5 69.0 71.0
Max. Sun 119.0 124.0 120.0 125.0 125.0 126.2
Greatest Diff. 72.0 78.0 60.0 66.0 65.0 62.2
Mean Max. Shade 47.2 50.0 58.4 63.7 65.3 66.7
Minim. Shade 29.0 25.5 37.0 38.0 38.0 51.5
Minim. Rad. 16.0 23.0 27.8 33.0 40.0 47.0
Greatest Diff. 12.7 15.3 8.7 16.0 10.0 4.8
Mean Minim. Shade 32.8 34.2 43.1 48.1 50.0 55.8
Mean Daily Range of Temp. 14.4 15.8 15.3 15.6 15.3 10.9
Sunk Therm. 46.0 48.0 50.0 58.0 61.0 62.0
Mean Dew-Point 34.3 37.2 45.8 49.8 54.4 59.5
Mean Dryness 5.1 3.9 5.8 6.6 2.7 2.0
Force of Vapour .216 .239 .323 .371 .434 .515
Pressure of Dry Air 23.091 .066 .084 22.909 .825 .692
Mean Saturation .84 .87 .82 .80 .91 .93
Rain in Inches 1.72 0.92 1.12 2.52 9.25 26.96

July Aug. Sep. Oct. Nov. Dec. Mean
Pressure of Atmosphere* 23.203 .230 .300 .372 .330 .365 23.289
Range of Pressure .062 .070 .082 .075 .078 .062 .074
Mean Shade 61.4 61.7 59.9 58.0 50.0 43.0 53.5
Max. Shade 69.5 70.0 70.0 68.0 63.0 56.0 65.4
Max. Sun 130.0 133.0 142.0 133.0 123.0 108.0 125.7
Greatest Diff. 62.0 62.0 70.0 65.0 68.0 77.2 67.3
Mean Max. Shade 65.5 66.1 64.7 66.5 56.5 51.6 60.2
Minim. Shade 56.0 54.5 51.5 43.5 38.0 32.5 41.3
Minim. Rad. 52.0 50.0 47.5 32.0 30.0 26.0 35.4
Greatest Diff. 3.5 3.5 10.0 12.0 12.0 10.0 9.9
Mean Minim. Shade 57.3 57.4 55.2 49.5 43.5 34.9 46.8
Mean Daily Range of Temp. 8.2 8.7 9.5 17.0 13.0 16.7 13.4
Sunk Therm. 62.2 62.0 61.0 60.0 55.0 49.0 56.2
Mean Dew-Point 60.7 60.4 58.5 52.5 46.5 31.8 49.4
Mean Dryness 0.8 1.1 1.4 4.2 3.2 10.6 4.0
Force of Vapour .535 .530 .498 .407 .331 .198 .383
Pressure of Dry Air 22.668 .700 .802 .865 .999 23.165 22.906
Mean Saturation .97 .96 .95 .86 .90 .69 .88
Rain in Inches 25.34 29.45 15.76 8.66 0.11 0.45 Sum
122.26

*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 Mr. Muller's, which is a little lower, and very near.

Horary Observations at Jillapahar, Dorjiling, Alt. 7,430 feet.