50 MR JOHN AITKEN ON DEW. 



Summary. 



Our principal conclusions may be summed up as follows : — First, From the 

 experiments made with (a) the inverted trays, (b) by weighing small areas of 

 turf, and (c) by observations of the temperatures on and under the grass 

 during dewy nights, we have concluded that vapour is almost constantly rising 

 from grass land, by night as well as day, in our climate. Second, From experi- 

 ments made with (a) inverted trays, (b) by weighings of small areas of soil, and 

 (c) by observations made with small test condensing surfaces, on dewy nights, 

 we have concluded that, under most conditions of our climate, vapour rises 

 from uncultivated areas of soil during night as well as clay. It follows from 

 these two conclusions that dew never " falls " on the earth : and for reasons 

 given, it is only deposited on plants, and other bodies, not in good heat com- 

 munication with the ground. Third, That the greater part of the dew condensed 

 on bodies near the ground is formed of the vapour rising at the time from the 

 earth, and very little of it from the vapour that rose during the day. Fourth, 

 That dew forms copiously on roads, but owing to the stones being good con- 

 ductors of heat, the vapour is deposited on the under sides of the stones, and 

 not on the top as on grass. Fifth, Wind hinders the formation of clew by 

 preventing an accumulation of clamp air near the ground. Sixth, The " dew- 

 drop " formed on grass and other plants is not dew at all, but is formed of the 

 exuded sap of the plant. Seventh, Almost all substances, such as black and 

 white cloths, garden mould and grass, radiate equally well at night. Among the 

 few exceptions observed are polished metals and sulphur. Eighth, A covering 

 of snow on the ground lowers the mean temperature of the air. 



FURTHER REMARKS ON DEW. 



Appendix. 



(Read 19th July 1886.) 



Since the preceding paper was written, a few opportunities have occurred 

 for continuing the investigation under different conditions of climate, and some 

 additions have also been made to strengthen the exudation theory of the " dew- 

 drop " on grass and other plants, of which I shall here give a short account. 



At the beginning of the paper, evidence is adduced to show that water 

 vapour is almost constantly rising from the ground during night, as well as day, 

 and it is there noted that the experiments were somewhat unsatisfactory on 

 account of the rather damp condition of the soil at the time, the dry season 

 being over before the investigation was made. In order to supplement these 

 observations, a few others were made in the beginning of July of this year, when 

 the soil was in about as dry a condition as it almost ever is in this climate. 



