26 MR JOHN A1TKEN ON DEW. 



with black varnish, ordinary glass mirrors, and also small black mirrors, in 

 order to get rid of the objection to ordinary silvered mirrors, namely, that they 

 might not be good radiators. On no occasion up to the beginning of November 

 have I yet seen dew on any of these at night, but it is difficult to say whether 

 dew had not formed on them on some mornings, as the air was thick and 

 misty, and the deposit then observed might have fallen as fine rain. 



The changes in temperature of the surface of the soil clue to radiation, give 

 rise to a downward movement of heat during the day, and to an upward move- 

 ment of it during the night. These heat changes will be accompanied by 

 corresponding movements of the moisture in the soil. During day, after the 

 surface is heated, the vapour tension being higher above than below, a 

 downward movement of moisture will take place ; and at night this process 

 will be reversed, the tension of the vapour at a depth being greater than near 

 the surface, the vapour rises and condenses in the colder soil. Part of the 

 latent heat so liberated by the rising vapour is spent in radiation from the 

 surface, part in evaporating moisture, and a little in heating the air cooled by 

 contact with cold grass, &c. 



We may conclude that, owing to the heat received during the day, and 

 probably also to the internal heat of the earth, vapour continues to rise from 

 the ground long after the sun has set, and in many conditions the vapour 

 continues to rise the whole night; but under certain others it seems probable 

 that the reverse will occasionally take place, and vapour condense on the 

 ground. This is most likely to take place soonest on bare soil, especially on 

 those parts of it that are in bad heat communication with the ground under- 

 neath. But over grass-land in most conditions of our climate, when dew is 

 forming, the evaporation seldom seems to stop, but goes on night and day, on 

 account of the surface of the soil being protected by the grass from losing its 

 heat so quickly as the bare soil. The escaping vapour rises till it meets with 

 some surface not in good heat communication with the ground, and which has 

 been cooled by radiation, in the manner set forth by Wells and others. 

 These remarks refer to weather when dew is most abundant, as in spring, 

 summer, and autumn, and do not apply to those conditions in which a warm 

 vapour-laden air is brought over a cold ground. 



Dew and Wind. 



It is well known that during windy nights no dew is formed. We pre- 

 viously knew that wind acts in two ways to prevent the formation of dew; 

 to these two ways we must now add a third. Wind prevents the formation of 

 dew — (1) by mixing the hot air above the surface of the ground with the air 

 cooled near its surface, this tends to prevent the air being cooled to the dew- 



