142 



MR. W. N. SHAW ON HYGROMETRIC METHODS. 



These instruments, however, may be regarded as being intended to facilitate the 

 reading of the dew-point rather than as questioning the results given by REGNAULT'S 

 instrument. The case is different with a new condensing hygrometer, introduced by 

 CROVA. * This consists of a tube of thin brass, the interior of which is nickel-plated 

 and carefully polished ; one end is closed by a plate of ground glass, and the other by 

 a lens. The air to be experimented on is drawn through this tube by means of two 

 tubulures entering it at right angles, and the tube is cooled by surrounding it with a 

 brass box, partly filled by carbon disulphide, through which air is made to pass. The 

 temperature is read by means of a thermometer dipping into the liquid. The dew 

 deposit is seen by means of the lens, and the arrangement is such that the mean 

 temperature of appearance and disappearance can be read to less than 0'l C. This 

 arrangement was adopted by CROVA, because he was dissatisfied with the behaviour 

 of REGNAULT'S instrument when the dew-point was very low, and when there was a 

 considerable wind. 



Having arranged the instrument, he made a series of comparisons of the results of 

 REGNAULT'S instrument, observed in the open air or window, with those of his own, 

 the air being in that case conducted through a fine lead tube to the instrument inside. 

 The thermometer employed had been previously corrected. 



The following are some of the results : 



SERIES I. Wind north-east, fairly strong. Barometer 759. 



SERIES II. Wind north-west, gradually increasirg. Barometer 758. 



humidity 0'54 to 0'58. 



Psychrometer 



* 'Journal do Physique,' vol. 2, 1883, p. 166. 



