108 Dr, Masons Description 



Leslie's Hygrometer 42°. 



Mason's Hygrometer 42°, no difference indicated. 



Experiment 10. — 4 P.M., Leslie's Hygrometer in the 

 shade 40° ; window shutters open so that the direct rays of 

 the sun fell upon the instrument. It descended to 106 

 degrees in ten minutes, being a variation of 66 degrees 

 under the same state of dryness. 



Those experiments were conducted with much care in a 

 large room having two windows on the same side, one of 

 which remained open during the whole of the experi- 

 ments. 



From these experiments it will be perceived that Sir 

 John Leslie's Hygrometer varied from 8 to 55 degrees, 

 whereas the Hygrometer by evaporation now proposed, 

 under the same circumstances, varied only 0*666 of a 

 degree. 



The above results were obtained in Madeira ; a second 

 Hygrometer by evaporation was kept in the shade during 

 the time to compare with the other. 



I regret I was not able to procure in the island two of 

 Leslie's Hygrometers, as the comparison may be thought 

 not so accurate ; but in all those variations, when the 

 causes of disturbance were removed, the Hygrometer 

 always indicated the same relative difference with my own, 

 as the observations at 6 A.M., 9 A.M., noon, and 3 P.M., 

 sufficiently prove. 



The following Table will enable the observer, at one 

 glance, to ascertain the equivalent indications afforded by 

 the three kinds of Hygrometers I have mentioned in my 

 paper, namely, the Dew-point Hygrometer, Sir John 

 Leslie's, and my own, which I have calculated for the 

 purpose. 



I have given the several results obtained by my own ex- 

 periments as far as I have carried them, and they may be 

 easily extended ; I may also add that the whole experi- 

 ments have been made from 40° to 90° of Fahrenheit ; my 

 experiments at lower temperatures are not yet sufficiently 

 extensive, and may form the subject of a future communi- 

 cation. 



To find by this Table the Dew-pointy corresponding to 

 the degree of dryness indicated by my Hygrometer, it is 



