Climate of London and Philadelphia. 13 



the absolute quantity of water which is contained in 

 the air, as upon the air being in a state of perfect or 

 imperfect solution. During long-continued summer 

 droughts, a very large proportion of water is dissolved 

 in the air ; notwithstanding this, the air is still dry, 

 and continues to be so as long as the water remains 

 in a state of perfect solution ; but no sooner are the 

 powers of solution diminished, than what was before 

 a dry, now becomes a moist climate. 



In the third instance, the dissolved water may be 

 cither slowly precipitated in the form of dew, or driz- 

 zling mist, or, more powerfully, in heavy rain ; or 

 there may be local and copious precipitations from 

 particular regions, as in sudden thunder-showers, 

 while other parts of the atmosphere retain their water 

 in a state of perfect solution. 



Agreeably to the above doctrine, which has been 

 ably supported by Dr. Halley, M. Le Roi, Dr. Dob- 

 son, and other eminent writers, it would appear, that 

 atmospheric air is the proper solvent of water, as 

 water is of salt, and, consequently, that it ought to 

 be considered as the immediate cause of evaporation. 

 But this theory, though ingenious and beautiful (as 

 it certainly is), will, nevertheless, I apprehend, on a 

 more close enquiry, be found unsatisfactory*, because 

 it has lately been discovered, by experiment, that 

 water evaporates more speedily in ■vacuo than in IheJ 



* For, notwithstanding the affinity between air and water, yet 

 it has never been proved, that the former alone can dissolve tlie 

 ' latter. 



