Himalayan Journals, vol 2 by J. D. Hooker (android pdf ebook reader TXT) 📕

unaffected by the diminished pressure. Any number of species from low countries may be cultivated, and some have been for ages, at 10,000

to 14,000 feet without change. It is the same with the lower animals; innumerable instances may with ease be adduced of pressure alone

inducing no appreciable change, whilst there is absence of proof to the contrary. The phenomena that accompany diminished pressure are

the real obstacles to the cultivation of alpine plants, of which cold and the excessive climate are perhaps the most formidable.

Plants that grow in localities marked by sudden extremes of heat and cold, are always very variable in stature, habit, and foliage. In a state of nature we say the plants "accommodate themselves" to these changes, and so they do within certain limits; but for one that

survives of all the seeds that germinate in these inhospitable

localities, thousands die. In our gardens we can neither imitate the conditions of an alpine climate, nor offer others suited to the

plants of such climates.

The mean height of the barometer at Mr. Hodgson's was 23.010, but

varied 0.161 between July, when it was lowest, and October, when it was highest; following the monthly rise and fall of Calcutta as to

period, but not as to amount (or amplitude); for the mercury at

Calcutta stands in July upwards of half an inch (0.555 Prinsep) lower than it does in December.

The diurnal tide of atmosphere is as constant as to the time of its ebb and flow at Dorjiling as at Calcutta; and a number of very

careful observations (made with special reference to this object)

between the level of the plains of India, and 17,000 feet, would

indicate that there is no very material deviation from this at any

elevation in Sikkim. These times are very nearly 9.50 a.m. and about 10 p.m. for the maxima, the 9.50 a.m. very constantly, and the 10

p.m. with more uncertainty; and 4 a.m. and 4 p.m. for the minima, the afternoon ebb being most true to its time, except during the rains.

At 9.50 a.m. the barometer is at its highest, and falls till 4 p.m., when it stands on the average of the year 0.074 of an inch lower;

during the same period the Calcutta fall is upwards of one-tenth of an inch (0.121 Prinsep).

It has been proved that at considerable elevations in Europe, the

hours of periodic ebb and flow differ materially from those which

prevail at the level of the sea; but this is certainly not the case in the Sikkim Himalaya.

The amplitude decreases in amount from 0.100 at the foot of the

hills, to 0.074 at 7,000 feet; and the mean of 132 selected

unexceptionable observations, taken at nine stations between 8000 and 15,500 feet, at 9.50 a.m. and 4 p.m., gives an average fall of 0.056

of an inch; a result which is confirmed by interpolation from

numerous horary observations at these and many other elevations,

where I could observe at the critical hours.

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.

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