212 Mr. J. Aitken on Dew. 



perhaps it would help to remove a difficulty some have felt in 

 accepting some of the conclusions contained in my paper. 

 One of the results contended for is, that dew on grass and on 

 bodies near the ground is formed from vapour rising from 

 the ground during night ; the reason given for this conclusion 

 being that grass-land is always in a condition to give off 

 vapour during dewy nights. The vapour that rises from the 

 ground after sunset will thus displace the vapour that rose 

 during the day, and the latter will diffuse itself into the drier 

 air over the grass. The stems and blades of the grass during 

 night will thus be surrounded by the vapour that has risen 

 most recently from the ground. Another reason given is, 

 that at night the temperature of the air among the stems is 

 much higher than that of the air at the tips of the blades, 

 and being in contact with moist soil is nearly saturated. The 

 vapour-tension of this hot air, rising among the stems, is thus 

 much higher than that of the air over them, and is thus in a 

 much more favourable condition for forming dew than the air 

 higher up. 



These are the reasons given for concluding that dew on 

 grass is formed from vapour rising at the time from the 

 ground and not from that which rose during the day. The 

 principal difficulty experienced by some in accepting this 

 conclusion, seems to have arisen from extending this con- 

 clusion to the source of the dew deposited on bodies placed 

 some distance from the ground. Now, when we consider 

 what takes place in these higher positions, it is easy to see 

 that the conditions are much more complicated, and we can 

 now say very little about the vapour, either as to the place of 

 its evaporation or as to the time when it changed its state. 

 Whenever the vapour coming from the ground rises above the 

 grass and mixes with the drier air over it, we get an entirely 

 different and much more complicated condition of matters. 

 No doubt some of the vapour molecules in this upper air will 

 have risen from the ground only a short time before, but some 

 of them will certainly, if there is the slightest wind, be mole- 

 cules which have risen during the day, and no doubt some of 

 them will have risen into the air many days previously ; and 

 while some will have risen from the ground immediately 

 underneath, others will have come from lands and oceans far 

 away. But while this may be so, it in no way affects the 

 conclusion that vapour is almost constantly, night and day, 

 given off by the soil, and that dew on grass is part of this 

 rising vapour trapped by the cold blades. 



Darroch, Falkirk, Yours trul y> 



June 14, 1886. JOHN AlTKEN. 



