54 MR JOHN AITKEN ON DEW. 



close to the ground, is formed of vapour rising from the ground, is that at 

 night the ground under the grass is always in a condition to give off vapour, 

 and this rising vapour will tend to displace that which rose during the day ; 

 the stems and blades of grass will thus become surrounded by vapour that 

 has risen during the night. A further reason is, that the ground under the 

 grass is much warmer than the air over it, and the air in contact with the 

 moist earth is nearly saturated ; the tension of the vapour in the air rising 

 through the stems of grass is thus higher than that in the air over them, and it 

 is therefore in a more favourable condition for condensing and forming dew 

 than the air higher up. This hot rising vapour will often indeed yield dew 

 when the air over the grass can give none. 



When, however, we come to consider what takes place higher up above the 

 grass, or even at the tops of the blades, we meet with a much more compli- 

 cated condition of matters, and we are now able to say very little about the 

 source -of the vapour condensed at these higher positions. Whenever we get 

 above the protection of the grass, into the parts of the atmosphere exposed to 

 air-currents, we can say very little as to the source of the vapour existing there, 

 either as to the place where it changed to vapour, or the time when this 

 change took place. No doubt some of the molecules in this upper air will 

 have risen but recently from the ground, but some of them will certainly — if 

 there is the slightest wind — be molecules that have risen during the day, and 

 no doubt some of them will have ascended into the air many clays previously ; 

 and while some will have but recently come from the ground immediately under- 

 neath, others will have travelled from lands and oceans far away. But while 

 this may be so, it in no way affects the conclusion that vapour is almost con- 

 stantly — night as well as day — given off' by the ground, and that dew on grass, 

 and on bodies close to the ground, is part of this rising vapour trapped by their 

 cold surfaces. 



Curiously enough, history here repeats itself. The theory that dew rises 

 from the ground has before now been wrecked by the observation that dew forms 

 on bodies placed high above the ground, and in situations where no vapour 

 could have risen to them from beneath. Professor Musschenbroek rejected 

 Gersten's theory of rising dew after he found dew was deposited on bodies 

 placed on the leaden roof of his observatory. He thought the dew formed 

 under those conditions could not have risen from the ground, but must have 

 fallen from the atmosphere. There must, therefore, evidently be a foundation 

 somewhere for the rejection of the theory on the grounds stated, though it must 

 be admitted it is difficult to find, as the statements are in no way opposed to 

 each other. It will, therefore, be as well for us to consider here the cause of 

 this appearance of opposition. 



One cannot help thinking that a good deal of the difficulty experienced in 



