Mil. W. N. SHAW ON UYGKOMETRIC METHODS. 



141 



humidity between the two chemical hygrometers, and between their mean and 

 RENAULT'S, the fractional humidity in each case being not far from '50 : 



These results are very satisfactorily concordant. The aspirator for the chemical 

 method contained 4 '278 litrtes, and the tubes were filled with sulphuric acid and 

 pumice, the air was taken from out of doors, and the REGNAULT, placed at the 

 window, was observed at the beginning, middle, and end of each experiment, and the 

 mean of the three dew-point readings taken. 



Observations were also taken with DANIELL'S instrument, but they were so dis- 

 cordant that they are not recorded. 



For many years REGNAULT'S form of condensation hygrometer was regarded as a 

 satisfactory standard instrument. In 1877 ALLUARD introduced a somewhat modified 

 form, which was used by ANGOT in the researches already discussed (p. 133). The 

 modification consisted in replacing the silver thirnble by a brass tube of square section 

 provided with various metal tubes to allow of the passage of air through the ether 

 contained in the tube. Instead of the glass upon which REGNAULT'S thimble was 

 mounted, there are two windows in opposite sides of the square tube, near to the top. 

 These enable the bubbling of the air through the ether to be watched. The sides of 

 the tube are gilt and highly polished, and one of them is framed by a broad band 

 of brass, gilt and polished in like manner. This surrounding band is very near to, 

 but does not touch, the brass tube. The dew is therefore deposited on a flat gilt 

 surface, and the identification of a deposit is rendered easier by the proximity of an 

 unaltered surface with which to compare the cooled one. 



A dew-point instrument was also introduced in 1871, by Mr. DINES, in which the 

 dew deposit was caused by cooled water, and took place on blackened glass ; this was 

 modified by its inventor in 1879,* so that it could be used with either water or ether. 



Sec Symons's " History of Hygrometer*," 'Meteorol. Soc. Quart. Jonrn.,' veil. 7, p. 161. 



