106 Dr, Masons Description 



exactly as many degrees as the obtained temperature exceeds 

 the absolute temperature of the shade. 



The degree of dryness on the thermometric scale will 

 be also equally increased, and thus indicate a fallacious 

 dryness. 



h. The comparative merits of the Hygrometer by evapo- 

 ration now proposed and Sir John Leslie's may be seen by 

 the following experiments : — 



When both instruments were in perfect shade, and free 

 from currents of air, my Hygrometer registered one-sixth 

 of Sir John Leslie's ; but it is only when Leslie's Hygro- 

 meter is in perfect shade and free from currents of air that 

 this relation exists between the instruments; indeed the 

 greatest nicety is required to make correct observations 

 with Sir John Leslie's Hygrometer as these experiments 

 will prove. 



Eperiment 1. — 6 A. M. Both instruments in the shade 

 and free from currents of air. 



Temperature of shade 6Q^. 



Moistened bulb . . 62|-°-difference 4i°x 6=27°. 



Leslie's Hygrometer 27°. 



Mason's Ditto . . 27°. no difference indicated. 



Eperiment 2. — 8 A.M. Windows and doors closed; in- 

 struments in the shade : the sun's rays fall upon one of the 

 white shutters, and are reflected upon the instruments at 

 three feet distance. 



Temperature of air 68°. 



Moistened bulb . 62^ difference 6° x 6=36°. 



Leslie's Hygrometer 45°. 



Mason's Ditto . . 36°— = 9° + Leslie's 

 Giving a false indication of nine degrees from the reflection 

 of the sun's rays from the white window shutter upon the 

 instrument. 



Experiments. Half-past 8 A,M. More of the shutter 

 exposed to the sun's rays. 



Temperature of the air . 68°. 



Ditto of the moistened bulb 62^°, difference 5i° x 6=33°. 



Leslie's Hygrometer 46^°. 



Mason's Ditto . . 33°.— = 13^°-^ Leslie's 

 Showing a variation of 13^ degrees from reflection. 



