1888.] S. A. Hill — Fsychrometer and Condensing Hygrometer, 371 



20 X 5280 



second tt^zt, = 29*33 cubic feet vv^ill come in contact with the 



ooOO 



given surface. Under ordinary circumstances the mass of a cubic foot 



of air is about 0'08 Bb., and its specific beat is 0'2875 ; therefore the 



total thermal capacity of the air which impinges on the square foot in 



one second is 29-33 x 0'08 X '2375 = 0-55. Let this air be cooled 



from 105. °2 F. to 30-7° F. This is the extreme case presented by the 



observations below, and is a much greater difference of temperature 



than any contemplated by Mr. Strachan. The air which reaches the 



plate every second will yield only (105-2— 30-7) x 0-55 = 40-975 ther- 



mal units. Supposing this heat to be all taken up and transmitted by 



the plate, it can only produce a difference of temperature between the 



40-975 

 two sides equal to = 0-0414° F, or only about -gV of a degree, 



yyu 



Now, wben experimenting in the open air, it is found impossible to 



determine the dew point with a degree of precision more minute than one 



or two tenths of a degree, however delicate the apparatus may be, as 



the dew point is constantly varying ; hence a source of error which can 



never under any circumstances actually occurring affect the observations 



to the extent of more than -^ of a degree may be safely neglected. 



An important practical difiiculty in the use of any form of conden- 

 sing hygrometer in very dry, hot weather, and one which renders the 

 use of Daniell's instrument impossible under such circumstances, is that, 

 without artificial cooling by some other means, it is almost impossible, 

 by blowing or aspiration, to make ether evaporate rapidly enough to 

 cool the liquid and the vessel which contains it down to the dew point, 

 and maintain them at or near that temperature for an appreciable 

 length of time. A more volatile liquid, like bisulphide of carbon, would 

 probably do better, but apart from the objection to the use of this liquid 

 on account of its evil smell, it cannot be used, because its fumes in- 

 stantly tarnish the brightly polished silver surface. In the Allahabad 

 observations 1 and 2 of the second series, tabulated below, this difiiculty 

 was got over by passing the current of air from the mouth through a 

 small flask packed round with crushed ice in a covered beaker, which 

 was placed about 18 inches from the hygrometer and on the leeward 

 side of it. The breath before bubbling through the ether was thus 

 cooled down almost to the dew point of the external air, and its excess 

 of moisture was removed by condensation in the flask. Such a method 

 of attaining the desired result would have been inadmissible in a place 

 altogether devoid of ventilation, but no objection to it can arise when 

 the instruments are directly exposed to our April hot winds. 



In all the observations printed in Table I., the thermometer readings 



