Mr. J. Aitken on Dew. 209 



beyond the bare statement I am not aware of anything he 

 advanced to support his idea. His theory thus fails to explain 

 the difference between the dew-drop on plants and true dew. 

 Nor does it account for dew on dead matter. On these his- 

 torical matters I am, however, open to conviction, as it is 

 extremely difficult to get access to all that has been written 

 on this very popular subject. 



As I nowhere say that vapour may not be condensed out of 

 air under conditions other than those where dew is forming, 

 the two paragraphs at the foot of page 487 have no bearing 

 on the subject. 



I am unable to see the relation between the observations 

 made by Wells and Six and myself, which Mr. Tomlinson 

 brings together by the footnote at page 488. In the investi- 

 gations made by the two former observers, the difference 

 remarked was between the readings of thermometers placed 

 on the grass and other substances, and thermometers hung 

 above them ; while in my experiments some thermometers 

 were placed on and others under the grass. The only like- 

 ness in the two sets of observations is a similarity in the 

 amount of the difference in the readings of the respective 

 sets of thermometers. 



It is unnecessary to follow Mr. Tomlinson through the 

 natural processes as described in pages 489, 490, further than 

 to say that the cooling he there describes as taking place on 

 grass at night is not correct. At the middle of page 490 he 

 says : — " We may imagine three layers of air — one in contact 

 with the points of the grass ; the second immediately below 

 it, where the blades are more numerous, and more or less ex- 

 posed to the zenith ; and the third entangled in the matted 

 portion, which is entirely sheltered from the sky." He then 

 describes the manner in which he supposes these layers to get 

 cooled, one after the other, beginning at the upper, till " in 

 the end the lower stratum will be colder than the first, so that 

 the blades and stems which are least exposed to the aspect of 

 the sky will be colder than the points of the blades, and the 

 thermometer buried in the grass will mark a lower tempera- 

 ture than one in contact with the surface/'' If Mr. Tomlinson 

 had read my paper carefully, or practically put the question 

 to nature, he would not have ventured such an opinion, as he 

 would not, I expect, ever have found the " thermometer buried 

 in the grass mark a lower temperature than the one in contact 

 with the surface." On the contrary, all through the night 

 the low r er thermometer remains warmer than the upper one. 

 Mr. Tomlinson, in describing the progress of cooling, appears 

 to have forgot to take into account the supply of heat given 



Phil, Mag. S. 5. Vol. 22. No. 135. August 1886. P 



