140 MR. W. N. SHAW ON HYGROMETRIC METHODS 



agrees with that author in preferring the chemical method, and recalls the attention 

 of meteorologists to an apparatus, described by him 17 years previously, intended to 

 give continuous observations by the chemical method. It is a sort of hydrometer 

 containing pumice and sulphuric acid, and is floated in an oil bath, and there is an 

 arrangement for passing the air through without interfering with the free flotation. 



A similar apparatus is described by SNELLEN.* I am inclined to look upon all 

 apparatus requiring anything like a long leading tube with very grave suspicion. A 

 sudden change in the temperature such as sometimes covers all walls and furniture 

 with a deposit of moisture would supply a layer of water to the leading tube that 

 would nearly saturate the air for a considerable time and render the instrument worse 

 than useless at a time when its results are wanted to be most accurate. Vox BAUM- 

 HAUER suggests leading air by means of a lead tube from a captive balloon to his 

 instrument in an observing room. It is almost painful to try and think what an 

 observation with such an arrangement would really mean. 



A short paper by LEFEBVREt gives the results of experiments to compare 

 REGNAULT'S hygrometer with the chemical method. One series of experiments com- 

 pares the hygrometric state as given by the chemical hygrometer with that deduced 

 from the dew-point reading of REGNAULT'S instrument, taking the dew-point to be : 



(1.) The temperature of the first appearance of dew. 

 (2.) The temperature of disappearance. 

 (3.) The mean of the two preceding. 



The collective results may be tabulated as follows, calling differences from the 

 results of the chemical hygrometer errors : 

 Number of observations, 1 5. 

 Range of temperature, 17'3 C. to 24 "0 C. 



f 2-00 in percentage humidity. 



Extreme error of first appearance observations < 



1 + '60 



1-83 



18 



T92 

 + '75 



Mean error of " first appearance " observations . '47 



"disappearance" . '74 



"mean" '55 



" disappearance " ,, < 



" mean ,, < 



A second set of experiments gives a comparison of two chemical hygrometers with 

 each other and with REGNAULT'S. I give a table of the differences in the fractional 



* 'Archives Ni'ei-landaisop,' vol. 9, p. 477. 

 t ' Annalcs de Chimie,' vol. 25, 1819, p- HO. 



