MR JOHN AITKEN ON DEW. 55 



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reconciling the statement, that the dew formed on bodies near the surface of 

 the earth is formed of vapour rising from the ground, with the fact that dew is 

 found on bodies high up in the air is caused by a want of clearness in our 

 ideas, or perhaps rather to a persistence of primitive ideas. Dew was in olden 

 times often spoken of as falling from the heavens, and even yet we talk of 

 falling dew. This expression is to a certain extent associated with the idea of 

 falling rain — a process in which the moisture passes from place to place through 

 the air, and falls on bodies exposed to it ; and many seem to think that if dew 

 comes out of the ground it should be found only on bodies exposed to the earth. 

 That in fact rising dew is the converse of the old falling dew, whereas clew is 

 only so much moisture taken by a cold surface from the store of vapour in the 

 air. This explanation of the difficulty does seem somewhat absurd, but we all 

 know that old habits of thought have a curious way of asserting themselves, 

 and it seems the only way of explaining a difficulty so many have felt. 



Let us picture an imaginary state of matters, in which all the conditions shall 

 be as simple as possible. Suppose the vapour to be constantly rising from the 

 ground, and that the air is absolutely still ; and further, let us imagine that the 

 vapour flows upwards through the air in a continuous stream, only varying in 

 velocity at different hours. At 6 p.m. we will suppose the molecules of vapour 

 that left the earth at 6 a.m. to have arrived at a height a. Let dew now begin 

 to form, then the moisture condensed at 6 p.m., on bodies at the height a, will 

 be vapour that rose from the ground at 6 a.m., and bodies at intermediate heights 

 will have the vapour on them that rose at the intermediate hours. At 6 a.m. 

 the following morning the vapour that rose at 6 on the previous evening will 

 be at an elevation h, and the dew forming on all bodies lower than b at this hour 

 will be vapour that rose during the night. So that in these ideal conditions, if 

 we knew the rate of ascent of the vapour, we could tell the hour at which the 

 vapour — condensed at any height — rose from the ground. 



If this imaginary condition of matters was correct, then there would be 

 some reason for expecting that dew would only be deposited on bodies placed 

 over such areas as yield vapour. In nature, however, the conditions are much 

 more complicated. The vapour does not flow upwards in a uniform stream, but 

 is mixed with the air by eddies and wind currents, and carried to bodies far 

 from where it rose ; so that while we can say something about the source of the 

 vapour condensed on bodies near the surface of the earth, yet the molecules in 

 the higher air have no history we can interpret. While the vapour rising from 

 the ground plays an important part in the phenomena of dew, as it is not 

 only the source of that formed on bodies near the ground, but it also increases 

 the amount deposited on bodies high up, yet the rising vapour is not essential 

 to its formation, as dew may be deposited even though the country for many 

 miles all round is dry and incapable of yielding any vapour. In such case 



