324 Daniell on a iieiv Hj/grometer. 



sequent augmentation of the elevation due to a given difl'er- 

 ence of atmospheric pressure. But the atmosphere is not in 

 fact of uniform composition ; the quantity of aqueous vapour, 

 one of its component parts, varies almost every hour of the 

 day. It is subject to sudden increase, and as sudden diminu- 

 tion : and, in its ascent to higher regions, follows a very dif- 

 ferent law from that of the permanent elastic fluids. The baro- 

 meter measures the total pressure of the compound atmosphere, 

 the hygrometer furnishes us with the means of estimating the 

 insulated pressure of that portion of it, which, is fluctuating in 

 quantity, and uncertain in composition ; by deducting the latter 

 from the former, we bestow upon the problem the necessary 

 condition of its assumed simplicity. In low latitudes this cor- 

 rection is of most particular importance, as the pressure and 

 quantity of the vapour is in some proportion to the heat. For ex- 

 ample, Capt. Webb, in his Memoir upon the Measurement of the 

 Himalaya Mountains, {Journal of the Royal Institution, Vol. VI. 

 p. 55,) informs us, that he computed the elevation of a number of 

 stations upon that lofty range, from the observation of a column of 

 mercury, compared with the mean height of the barometer at Cal- 

 cutta in the same season. Now the thermometer, in the latitude of 

 the latter place, generally ranges throughout the year between 75° 

 and 95°, and oftenrises to 100°, and sometimes to 110°. We shall, 

 therefore, be probably under the mark in assuming the tempera- 

 ture of Calcutta, during Captain Webb's observation, at 80°. The 

 summits of the Himalaya mountains are above the limits of per- 

 petual congelation ; therefore, we cannot be much in error in 

 fixing the temperature of these higher stations at 32°. We will 

 next suppose that the air at Calcutta was not saturated with 

 moisture, but that the point of condensation was 10° below the 

 temperature of the air, while, on the mountain, it was at its 

 highest limit. The column of mercury which the former would 

 support would be 0.721 in., while that which would counterba- 

 lance the latter, would only be 0.200 in., making a difference of 

 0.521 in. to be deducted from the height of the barometer at the 

 lower station, and amounting to an error of 468 feet in the 

 estimated height of the upper. This, of course, as far as regards 



