relatitig to Hygrometers and Evaporation. ftl 



On comparing this with Mr Anderson's results, Edinburgh 

 Encyclopcedia, art. Hygrometry, a remarkable disagreement will 

 be perceived, both as to the quantity of the depression, and the 

 rate at which it is influenced by pressure. The following are 

 the results which Mr Anderson obtained by placing Leslie's hy- 

 grometer under a receiver along with sulphuric acid ; the tem- 

 perature of the air being 48°.5 Fahrenheit. 

 Inches. 



The first column is the pressure ; the second the depression 

 in degrees of Leslie's hygrometer, which, for the sake of com- 

 parison, I have reduced to degrees of Fahrenheit in the third. 



The temperature and pressure in Mr Anderson's first case, 

 are nearly the same as mine, but our depressions are very dif- 

 ferent ; his being only 4°.86, whilst mine is 11°. 6, which is 

 more than twice as great. This discordance led me, at first, to 

 suspect, that as, in Mr Anderson's experiments, the wet ball of 

 the hygrometer would, from its construction, be six or eight 

 inches above the surface of the acid, whilst in mine it did not 

 exceed one inch, this might be the reason why his depressions 

 were so small. But on trying this, the result was 49° — 39°=10°, 

 still double of Mr Anderson's numbers ; even though the sur- 

 face of sulphuric acid did not exceed half of that in the former 

 experiments, so that this does not appear to have been the rea- 

 son why Mr A.'s numbers are so small. Indeed I have repeat*, 

 edly obtained greater depressions than 4°. 86, by merely suspend- 

 ing the instrument in a room where no means were used to dry 

 the air, or raise its temperature * ; such as 46°.5 — 40°.5rz6°; 

 45°8.— 40.°5=5°.3 ; 47°.5— 41°= 6°.5. The barometer was 

 rather higher than Mr Anderson's ; and had the temperature 

 been raised to 48°.5 Fahrenheit, the depression would have been 

 a little increased. 



The difference surely could not proceed from any defect in 

 > 



* For, as is well known, very cold air, by being heated, without additional 

 moisture, becomes comparatively dry. 



