Himalayan Journals, vol 2 - J. D. Hooker (bill gates best books txt) 📗

and

clumps of planted Casuarina.

On the 28th of January we passed Saugor island, and entered the

Hoogly, steamed past Diamond Harbour, and landed at the Botanic

Garden Ghat, where we received a hearty welcome from Dr. Falconer.

Ten days later we bade farewell to India, reaching England on the

25th of March, 1851.

APPENDIX. A.

METEOROLOGICAL OBSERVATIONS IN BEHAR, AND IN THE VALLEYS OF THE SOANE

AND GANGES.

Most of the instruments which I employed were constructed by Mr.

Newman, and with considerable care: they were in general accurate,

and always extremely well guarded, and put up in the most portable

form, and that least likely to incur damage; they were further

frequently carefully compared by myself. These are points to which

too little attention is paid by makers and by travellers in selecting instruments and their cases. This remark applies particularly to

portable barometers, of which I had five at various times. Although there are obvious defects in the system of adjustment, and in the

method of obtaining the temperature of the mercury, I found that

these instruments invariably worked well, and were less liable to

derangement and fracture than any I ever used; the best proof I can give of this is that I preserved three uninjured during nearly all my excursions, left two in India, and brought a third home myself that had accompanied me almost throughout my journey.

In very dry climates these and all other barometers are apt to leak, from the contraction of the box-wood plug through which the tube

passes into the cistern. This must, in portable barometers, in very dry weather, be kept moist with a sponge. A small iron bottle of pure mercury to supply leakage should be supplied with every barometer, as also a turnscrew. The vernier plate and scale should be screwed, not soldered on the metal sheath, as if an escape occurs in the

barometer-case the solder is acted upon at once. A table of

corrections for capacity and capillarity should accompany every

instrument, and simple directions, etc., in cases of trifling

derangement, and alteration of neutral point.

The observations for temperature were taken with every precaution to avoid radiation, and the thermometers were constantly compared with a standard, and the errors allowed for. The maximum thermometer with a steel index, I found to be extremely liable to derangement and very difficult to re-adjust. Negretti's maximum thermometer was not known to me during my journey. The spirit minimum thermometers again, are easily set to rights when out of order, but in every one (of six or seven) which I took to India, by several makers, the zero point

receded, the error in some increasing annually, even to -6 degrees in two years. This seems due to a vaporisation of the spirit within the tube. I have seen a thermometer of this description in India, of

which the spirit seemed to have retired wholly into the bulb, and

which I was assured had never been injured. In wet-bulb observations, distilled water or rain, or snow water was used, but I never found

the result to differ from that obtained by any running fresh water, except such as was polluted to the taste and eye.

The hours of observation selected were at first sunrise, 9 a.m.,

3 p.m., sunset, and 9 p.m., according to the instructions issued to the Antarctic expedition by the Royal Society. In Sikkim, however,

I generally adopted the hours appointed at the Surveyor General's

office, Calcutta; viz., sunrise, 9h. 50m. a.m., noon, 2h. 40m. p.m., 4 p.m., and sunset, to which I added a 10 p.m. observation, besides many at intermediate hours as often as possible. Of these the 9h.

50m. a.m. and 4 p.m. have been experimentally proved to be those of the maximum and minimum of atmospheric pressure at the level of the sea in India, and I did not find any great or marked deviation from this at any height to which I attained, though at 15,000 or 16,000

feet the morning maximum may occur rather earlier.

The observations for nocturnal (terrestrial) radiation were made by freely suspending thermometers with naked bulbs, or by laying them on white cotton, wool, or flannel; also by means of a thermometer placed in the focus of a silvered parabolic reflector. I did not find that the reflector possessed any decided advantage over the white

cotton: the means of a number of observations taken by each

approximated closely, but the difference between individual

observations often amounted to 2 degrees.

Observations again indicative of the radiation from grass, whether

dewed or dry, are not strictly comparable; not only does the power of radiation vary with the species, but much more with the luxuriance

and length of the blades, with the situation, whether on a plane

surface or raised, and with the subjacent soil. Of the great effect of the soil I had frequent instances; similar tufts of the same

species of grass radiating more powerfully on the dry sandy bed of

the Soane, than on the alluvium on its banks; the exposure being

equal in both instances. Experiments for the surface-temperature of the soil itself, are least satisfactory of any:--adjoining localities being no less affected by the nature, than by the state of

disintegration of the surface, and by the amount of vegetation in

proximity to the instrument.

The power of the sun's rays in India is so considerable, and

protracted through so long a period of the day, that I did not find the temperature of springs, or of running water, even of large deep rivers, so constant as was to be expected.

The temperature of the earth was taken by sinking a brass tube a yard long in the soil.

A thermometer with the bulb blackened affords the only means the

traveller can generally compass, of measuring the power of the sun's rays. It should be screened or put in a blackened box, or laid on

black wool.

A good Photometer being still a desideratum, I had recourse to the

old wedge of coloured glass, of an uniform neutral tint, the distance between whose extremes, or between transparency and total opacity,

was one foot. A moveable arm carrying a brass plate with a slit and a vernier, enables the observer to read off at the vanishing point of the sun's limb, to one five-hundredth of an inch. I generally took

the mean of five readings as one, and the mean of five of these again I regarded as one observation; but I place little dependence upon the results. The causes of error are quite obvious. As far as the effects of the sun's light on vegetation are concerned, I am inclined to

think that it is of more importance to register the number of

hours or rather of parts of each hour, that the sun shines, and its clearness during the time. To secure valuable results this should be done repeatedly, and the strength of the rays by the black-bulb

thermometer registered at each hour. The few actinometer observations will be found in another part of the Appendix.

The dew-point has been calculated from the wet-bulb, by Dr. Apjohn's formula, or, where the depression of the barometer is considerable, by that as modified by Colonel Boileau.* [Journal of Asiatic Society, No. 147 (1844), p.135.] The saturation-point was obtained by dividing the tension at the dew-point by that at the ordinary temperature, and the weight of vapour, by Daniell's formula.

The following summary of meteorological observations is alluded to at vol. i., chapter i.

I.--_Table-land of Birbhoom and Behar, from Taldanga to Dunwah.

Average elevation 1,135 feet._

It is evident from these observations, that compared with Calcutta, the dryness of the atmosphere is the most remarkable feature of this table-land, the temperature not being high; and to this, combined

with the sterility of the soil over a great part of the surface, must be attributed the want of a vigorous vegetation. Though so favourably exposed to the influence of nocturnal radiation, the amount of the

latter is small. The maximum depression of a thermometer laid on

grass never exceeded 10 degrees, and averaged 7 degrees; whereas the average depression of the dew-point at the same hour amounted to 25

degrees in the morning. Of course no dew was deposited even in the

clearest star-light night.

February 1848.

Hour Sunrise 9 a.m. 3 p.m. 9 p.m.

TEMPERATURE

Mean 56.6 70.1 75.5 61.7

Max. 65.2 77.0 81.7 66.2

Min. 46.3 61.2 65.2 55.5

Range 18.9 15.8 16.5 10.7

WET-BULB

Mean 48.2 53.7 55.3 49.3

Max. Depression 12.5 19.3 22.5 20.5

Min. Depression 6.0 14.3 16.7 9.0

Elasticity of Vapour .276 .264 .248 .248

DEW-POINT

Mean 39.5 37.9 36.0 36.1

Max. 52.0 52.7 46.8 50.0

Min. 23.3 24.5 24.3 *9.1

Max. Depression 31.7 39.2 48.4 56.9

Min. Depression 10.4 24.3 34.9 16.2

Weight of Vapour in cubic feet 3.088 2.875 2.674 2.745

SATURATION

Mean