MR JOHN AITKEN ON DEW. 11 



clear and calm evening, is colder than the upper, and will, therefore, be sooner 

 in a condition to deposit a part of its moisture. 2. It is less liable to agitation 

 than the upper. 3. It contains more moisture than the upper, from receiving 

 the last which has risen from the earth, in addition to what it had previously 

 possessed in common with other parts of the atmosphere." Then he goes on to 

 give reasons why vapour cannot be rising out of the ground, but adds, that some 

 of it must be from this source, as bodies near the surface of the ground get dewed 

 sooner than those higher up, though equally cold with them, but says, " the 

 quantity from this cause can never be great," and proceeds to give his reasons, 

 which are not altogether satisfactory, and need not be quoted here. He then 



sums up as follows : — " These considerations warrant me to conclude 



that on nights favourable to the production of dew, only a very small part of 

 what occurs is owing to vapour rising from the earth ; though I am acquainted 

 with no means of determining the proportion of this part to the whole." 



I shall now proceed to detail the observations which have caused me to 

 differ from the conclusion so distinctly set forth by Dr Wells in the above 

 quotations. I need not say that all my experiments only confirm the con- 

 clusions of that observer as to the formation of dew — that is, as to the 

 conditions most favourable for the deposition of moisture on the surfaces of 

 bodies during dewy nights, while the earth is radiating heat into space. The 

 point on which we differ is as to the source of the vapour that condenses 

 on the radiating surfaces — -a point which Dr Wells admits there were no 

 facts to determine, his own opinion being formed by experiments that did not 

 bear directly on the subject. 



When I began to doubt the truth of the generally received opinion as to the 

 source of the vapour, I found a difficulty in beginning my investigation, as it 

 was not easy to arrange experiments to give a direct answer to the question. 

 My intention at first was to test, by means of a delicate hygrometer, the 

 humidity of the air at different heights from the ground and under different 

 conditions. This plan had, however, soon to be abandoned, owing to the 

 impossibility of making anything like accurate observations with any 

 instruments at present in use. 



For some time I have had in my possession a hair hygrometer constructed 

 by Chevallier of Paris, This form of instrument is perhaps one of the best 

 for the purpose ; yet on making a few test experiments with it, for the special 

 purpose under consideration, its indications were found to be nearly valueless. 

 For instance, if the instrument was removed from saturated to drier air, and 

 again replaced in the saturated, it was impossible to get the pointer back 

 again to the same position on the scale ; and as the amount of dryness it would 

 be required to measure was a very small degree removed from saturation, the 

 error in the indications might be greater than the actual amount of dryness. 



