The equivalent thus deduced is far greater than that brought out by the Sikkim observations. It indicates a considerably higher temperature of the atmosphere, and is probably attributable to the evolution of heat during extraordinary rain-fall, and to the formation of the surface, which is a very undulating table-land, and everywhere traversed by broad deep valleys, with very steep, often precipitous flanks; these get heated by the powerful sun, and from them, powerful currents ascend. The scanty covering of herbage too over a great amount of the surface, and the consequent radiation of heat from the earth, must have a sensible influence on the mean temperature of the summer months.
J.
ON THE MEASUREMENT OF ALTITUDES BY THE BOILING-POINT THERMOMETER.
The use of the boiling-point thermometer for the determination of elevations in mountainous countries appearing to me to be much underrated, I have collected the observations which I was enabled to take, and compared their results with barometrical ones.
I had always three boiling-point thermometers in use, and for several months five; the instruments were constructed by Newman, Dollond, Troughton, and Simms, and Jones, and though all in one sense good instruments, differed much from one another, and from the truth. Mr. Welsh has had the kindness to compare the three best instruments with the standards at the Kew Observatory at various temperatures between 180° and the boiling-point; from which comparison it appears, that an error of l·5° may be found at some parts of the scale of instruments most confidently vouched for by admirable makers. Dollond’s thermometer, which Dr. Thomson had used throughout his extensive west Tibetan journeys, deviated but little from the truth at all ordinary temperatures. All were so far good, that the errors, which were almost entirely attributable to carelessness in the adjustments, were constant, or increased at a constant ratio throughout all parts of the scale; so that the results of the different instruments have, after correction, proved strictly comparable.
The kettle used was a copper one, supplied by Newman, with free escape for the steam; it answered perfectly for all but very high elevations indeed, where, from the water boiling at very low temperatures, the metal of the kettle, and consequently of the thermometer, often got heated above the temperature of the boiling water.
I found that no confidence could be placed in observations taken at great elevations, by plunging the thermometer in open vessels of boiling water, however large or deep, the abstraction of heat from the surface being so rapid, that the water, though boiling below, and hence bubbling above, is not uniformly of the same temperature throughout.
In the Himalaya I invariably used distilled, or snow or rain-water; but often as I have tried common river-water for comparison, I never found that it made any difference in the temperature of the boiling-point. Even the mineral-spring water at Yeumtong, and the detritus-charged glacial streams, gave no difference, and I am hence satisfied that no objection can be urged against river waters of ordinary purity.
On several occasions I found anomalous rises and falls in the column of mercury, for which I could not account, except theoretically, by assuming breaks in the column, which I failed to detect on lifting the instrument out of the water; at other times, I observed that the column remained for several minutes stationary, below the true temperature of the boiling water, and then suddenly rose to it. These are no doubt instrumental defects, which I only mention as being sources of error against which the observer must be on the watch: they can only be guarded against by the use of two instruments.
With regard to the formula employed for deducing the altitude from a boiling-point observation, the same corrections are to a great extent necessary as with barometric observations: if no account is taken of the probable state of atmospheric pressure at the level of the sea at or near the place of observation, for the hour of the day and month of the year, or for the latitude, it is obvious that errors of 600 to 1000 feet may be accumulated. I have elsewhere stated that the pressure at Calcutta varies nearly one inch (1000 feet), between July and January; that the daily tide amounts to one-tenth of an inch (=100 feet); that the multiplier for temperature is too great in the hot season and too small in the cold; and I have experimentally proved that more accuracy is to be obtained in measuring heights in Sikkim, by assuming the observed Calcutta pressure and temperature to accord with that of the level of the sea in the latitude of Sikkim, than by employing a theoretical pressure and temperature for the lower station.
In the following observations, the tables I used were those printed by Lieutenant-Colonel Boileau for the East India Company’s Magnetic Observatory at Simla, which are based upon Regnault’s Table of the ‘Elastic Force of Vapour.’ The mean height of the barometrical column is assumed (from Bessel’s formula) to be 29·924 at temp. 32°, in lat. 45°, which, differing only ·002 from the barometric height corresponding to 212° Fahrenheit, as determined experimentally by Regnault, gives 29·921 as the pressure corresponding to 212° at the level of the sea.