on a new Theory of Dew, 485 



labours. Nevertheless, most of Wells's results had been pub- 

 lished long before the author commenced his labours, and the 

 theory for which he has obtained so much credit was also 

 similarly indicated, in brief but unmistakable terms *. 



The chief points which Dr. Wells is said to have established 

 may be thus stated : — 



I. That on clear and serene nights the surface of the earth 

 is colder than the air some feet above it. 



II. That on such nights dew or hoar-frost is formed. 



III. That in cloudy weather the temperature of the ground 

 approaches, and is often identical with, that of the air ; and 

 under such circumstances little or no dew is formed. 



IV. That screens, even of the lightest material, interposed 

 between the ground and the clear sky, and, in general, what- 

 ever interrupts the view of the sky, prevents that portion of 

 the ground thus protected from cooling below the temperature 

 of the air. 



V. That different bodies exposed to the clear sky become 

 colder than the air ; the times and amounts of cooling being, 

 in general, different in different bodies. 



VI. That all these varied phenomena are to be accounted for 

 on the principles of radiation and condensation, by the first 

 of which the surface of the earth after sunset, provided the 

 sky be clear, cools down below the temperature of the air ; 

 and by the second of which, the vapour suspended in the air 

 is reduced to the liquid state by contact with a body cooler 

 than itself. But should the sky be clouded or the ground be 

 protected by means of screens, the heat radiated from the earth 

 is reflected back again, and thus maintains the surface at or 

 about the same temperature as that of the air. 



Mr. Aitken, in his theory of dew, states : — 



I. That the ground below the surface is always hotter than 

 the air over it, and that, so long as this excess keeps the 

 surface above the dew-point of the air, it will, if moist, give 

 off vapour, which w v ill condense on the grass and form dew, 

 and not the vapour that was previously present in the air. 



II. That vapours rising from the ground during dewy 

 nights are thus trapped by the herbage, or within tin trays 

 inverted on the ground. 



III. That on weighing a small area of the exposed surface 



* The details are given in my essay, " On the Claim of Dr. Wells to be 

 regarded as the Author of the Theory of Dew," published in the ' Edin- 

 burgh New Philosophical Journal/ for January 1801 ; and in a somewhat 

 enlarged form in ' Experimental Essays/ published in 1863, in Weale's 

 Series. See also a notice from my pen in the ' Chemical News ' of the 

 12th April, 1867, of a reprint of Wells's Essay. 



Phil. Mag. S. 5. Vol. 21. Ko. lo3. June 1886. 2 M 



