1876.] H. F. Blanford — Experiments with the Psychrometer. 55 



(with small pea-bulbs) which Mr. Pogson had verified by comparison with a 

 Kew standard in his possession. 



The Upper Indian series (16 to 21) of psychrometric observations were 

 made with a pah' of Casella's thermometers of the Kew pattern (with small 

 spherical bulbs) mounted, 6 inches apart, on a portable wooden stand, in 

 such manner that the free access of air is unimpeded in all directions. 

 Both thermometers have been compared in water with my Kew standard 

 and have also been verified at the freezing point. The wet bulb was covered 

 with a single thickness of old thin calico ; the water bottle was placed 2 

 inches to the side of the bulb, with the water level |- inch below it, and 

 communication was made by a well-washed lamp-wick of some dozen 

 threads of coarse yarn. Care was taken that the bulb was at all times well 

 moistened. 



The psychrometer observations at Trichinopoly, Madras, and Calcutta 

 were taken with a sling thermometer, viz. that of the hygrometer, in 

 the intervals of the dew-point observations. 



In most cases the readings of the instruments were made by two ob- 

 servers, one of whom (myself) read off the Regnault, and the other the 

 psychrometer, on the dew-point signal being given by the first observer. 

 In very dry states of the atmosphere, the mercury of both the dry and wet 

 bulbs is in a state of constant oscillation, through a range of a degree or 

 more ; moreover, unless the silver capsule is very highly burnished and free 

 from microscopic scratches, there is considerable difficulty in seizing the 

 exact temperature at which dew appears, since the quantity deposited is 

 very small and but slightly dulls the surface even at 3 or 4° below the dew- 

 point. The surface of the capsule having been polished Avith plate-powder, 

 was not in the best condition, and it is likely that some of the discrepancies 

 to be noticed in the tables may have been in some measure due to this ; 

 but I do not think that the error thus arising could exceed a few tenths of 

 a degree, as great watchfulness was exerted, and any observation that appear- 

 ed doubtful at the time was rejected. I shall in the sequel suggest some 

 precautions and improvements which may be useful to future observers. 



In the following Tables, the dew-point determined directly by the 

 hygrometer is compared with that computed from simultaneous observa- 

 tions of the psychrometer by each of the three methods in common use. 

 The first is August's formula as corrected by Regnault and adapted to Eng- 

 lish standards. It is given at page 47 of Gruyot's Hygrometric Tables, for 

 wet-bulb temperatures above the freezing point as follows : 



0-480 x | (t-f) 0-480 g-Q 



J 610—| (*'— 32) J 1130— f 



wherein x is the tension of saturated vapour at the temperature of the dew- 



