1888.] S. A. Hill — PsycTirometer and Condensing Hygrometer. 369 



XV. — The Psychrometer and the Condensing Hygrometer. — By S. A. Hill, 

 B. Sc, Meteorological Reporter to the Government of the North-Westem 

 Provinces and Oudh. 



[Received August 23rd ;— Eead November 7th, 1888.] 



In continnation of his classical researches into the thermal pro- 

 perties of aqueous vapour, Regnault turned his attention to the subject 

 of hjgrometry, and a translation of his paper on this subject will be 

 found in Taylor^s Scientific Memoirs,, Vol. IV. The original paper in 

 the Comptes Rendus is, I believe, not accessible in India, or at all events, 

 in Allahabad. As the outcome of his researches he gave to the world 

 a perfected form of the chemical or absorption hygrometer, a new and 

 improved variety of condensing hygrometer, and an improved form,ula 

 for reducing the readings of the psychrometer, or combination of dry 

 and wet-bulb thermometers used as an instrument for determining the 

 degree of moisture of the air. 



The chemical hygrometer remains much as Regnault left it, but 

 various other forms of condensing or dew point instruments have since 

 been invented, though they are not much, if at all, known, in this 

 country. The best of these are two invented by AUaard and Crova, 

 Regnault's countrymen, and both of them are constructed on the same 

 principle as his, viz., that of cooling down a polished metallic vessel, by 

 the evaporation of ether or some other volatile liquid inside it, until dew 

 begins to be deposited on the surface, and noting the temperature at 

 which this effect occurs by means of a thermometer immersed in the 

 liquid. The chief difficulty in the use of Regnault's form of this instru- 

 ment is that the small silver capsule (now generally replaced by an 

 electro-plated one of thin brass) in which the etber is evaporated is diffi- 

 cult to maintain in as high a state of polish as is desirable, owing not only 

 to its liability to be scratched, but to the tendency of the silver to become 

 tarnished by accidental overflows of the ether. The consequence of a 

 loss of polish from any cause is that dew is not observed until the tem- 

 perature has fallen somewhat below the proper dew point. In Alluard'a 

 instrument, this difficulty is supposed to be got over by substituting for 

 the silver surface one of gilt brass, which is much less liable to tarnish, 

 and in Crova's instrument, by making the silver vessel in the form of a 

 hollow cylinder with a horizontal axis, on the inner surface of which 

 cylinder dew is observed by looking through it parallel to the axis. 

 Even with Regnault's original instrument the difficulty is not a serious 

 one, if care be taken to have the vessel properly burnished to start with, 

 to repolish it with fine rouge immediately before each series of obser- 

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