32 JDr, Masons Description 



but I have proved, which is sufficient for our purpose, that 

 the relative difference indicated between the dry and moist- 

 ened bulb of a thermometer is equal ; or in other words, 

 that the refrigerating process is the same under given 

 states of dryness or humidity : whether the thermometer is 

 in the shade or exposed to the direct rays of the sun, pro- 

 vided the instrument be suspended in free space at some 

 distance from the ground. 



This I have sufficiently proved to be the case, and can with 

 confidence assert that the relation between two hygrometers 

 of the above construction is constantly equal in sunshine 

 and in shade. I may instance two observations to elucidate 

 the matter, one under the ordinary state of the atmosphere, 

 the other during a Leste or dry wind. The hygrometer 

 in the shade stood as follows : 



Temperature of the dry bulb 75°. 



Temperature of the moistened bulb 68°, 7° dryness. 

 Hygrometer in the sun. 



Temperature of the dry bulb 83°. 



Temperature of the moistened bulb 76°, 7° dryness. 



The thermometer on the ground indicated 142°. 



During a strong Leste the hygrometer in the shade stood 

 as follows : — 



Temperature of the dry bulb 86°. 



Temperature of moistened bulb QQI^ 20. 

 Hygrometer in the sun. 



Temperature of the dry bulb 96°. 



Tempetature of the moistened bulb 72 and 96°— 72° =24° 

 ■ — 4° correction for strong wind =20; themometer on the 

 ground indicated 124°. Thus the influence of the sun affects 

 each thermometer equally, and the temperature produced 

 by evaporation is the same in both cases, being regulated by 

 the elasticity of the aqueous vapour already existing in the 

 atmosphere. Mr. Daniell makes the same remark on this 

 subject with regard to his dew-point hygrometer. I find 

 in looking over my observations on this subject, that the 

 maximum point of variation in the moistened bulb for the 

 same day at Madeira appears to be about 2°; frequently 

 it remains stationary ; but hitherto,. I have not been able 

 to trace the cause of this variation; I may, however, re- 

 mark, that this is greatest at the commencement of the 



