40 Harvey on the Deposition of Dew. 



the temperature of the glass was 70°, and of the air 69°, when no 

 moisture was perceived ; but in the space of half an hour, the 

 former indicated 67°, and the latter 66°, at which moment dew 

 was clearly perceptible; agreeing precisely with the discovery of 

 Dr. Wells, that bodies become colder than the neighbouring air 

 before they are dewed. 



The sun at six P.M. had an elevation of 17 degrees, but its di- 

 rect rays did not reach the place of observation. The sky also was 

 tranquil and unclouded. It was not, however, till a quarter after 

 seven, that dew was perceptible on the glass elevated 16 inches 

 above the field, an instance of the slowness with which the cooling 

 power of the land imparts its influence to the neighbouring air. At 

 a quarter before eight, just as the solar orb was disappearing, the 

 temperature of a glassy surface, 30 inches high, was sufficiently 

 low to obtain moisture from the air reposing on it. By sunset, 

 masses of wool, of twelve grains each, placed on the metallic plates 

 resting on the grass, and at the respective heights of 16 and 30 

 inches, received increments of dew amounting to 4, 3§, and 3 

 grains. Dew, therefore, is deposited in shady places, long before 

 sunset. 



In consequence of the interposition of a dense cloud in the 

 zenith, from half past nine to half past ten, the temperature of the 

 grass was elevated two degrees and a half, and that of the air, half 

 a degree, as may be seen by a reference to the table. The effect 

 of amass of clouds in elevating the temperature of the land, was 

 first observed by Mr. Wilson, of Glasgow, and afterwards con- 

 firmed by Dr. Wells. At the present time, however, it was re- 

 marked, that the appearance of the cloud was accompanied by an 

 almost entire suspension of dew ; and the minute quantity depo- 

 sited probably owed its origin to the clear horizon. In the inter- 

 val between a quarter past five, and half past nine, a mass of wool, 

 of 12 grains, received an increment of six grains and a half; but 

 from the last observation, to half past ten, the quantity gained was 

 only half a grain ; and its almost total suspension could be attri- 

 buted to no other cause than the appearance of the cloud ; since 

 the air preserved its tranquillity, and every other circumstance, ex- 



