HUMIDITY OF THE AIR. 153 



Mists, on the other hand, afford a direct indication that the 

 air at the place and time of their occurrence is loaded with 

 moisture in this sense absolutely humid but they happen 

 neither so regularly nor so frequently in Yorkshire as to yield 

 more than slight characters for its general climate. They, how- 

 ever, mark some periodical recurrences of particular conditions 

 which deserve attention. 



The most important information on the state of moisture in 

 the air, is to be had by experiment on the evaporation which takes 

 place from wet surfaces, and the deposition of dew on bodies 

 which are cooled below the temperature of the air ; and these 

 are precisely the observations which till within a few years were 

 much, and almost generally, neglected in England. 



We had in fact no satisfactory basis to proceed upon, till 

 Dalton's sagacity and Daniell's inventiveness gave us the Dew- 

 point Hygrometer, and Apjohn's calculations showed us how to 

 determine the degree of moisture in the air, from the mere power 

 of cooling which is well known to accompany evaporation. 



The average depth of the rain which falls in a year at York is 

 found to be nearly 24 inches, the greatest observed depth being 

 32-12 inches (in 1812), the least being 15-93 inches (in 1834). 



This quantity is pretty nearly the average of what falls on the 

 eastern or flatter side of the island, from London, which receives 

 24-80* inches, to Edinburgh, which registers 23-50*. The 

 quantity received near the eastern coast is perhaps generally less, 

 as at Keyingham 18 inches; and that measured farther inland, 

 or toward the western mountains, is always greater, the augment- 

 ation being the more marked the farther we go to the west and 

 north-west, till round the Lake district it becomes above twice, 

 and in certain parts of it six times as much as at York (see 

 Mr. Miller's researches in the Phil. Trans.). 



At Settle the mean annual depth being 43'3, from 1837 to 



* The numbers marked thus are from Sir J. Clark's interesting work on 

 Climate. 4th edition. 



