Mr. J. Aitken on Dew. 211 



touching them, that is without lifting them from the ground ; 

 whereas it is the condition of the undersides or insides of the 

 trays that are specially referred to. The slate and weight 

 resting on the ground were of course quite wet on their under- 

 sides, which were invisible except when moved, while their 

 exposed parts were dry ; whereas the elevated slate and weight 

 were wet all over. With this explanation I have no doubt 

 Mr. Tomlinson will see the importance of the contact with the 

 ground in keeping the temperature of the slate and weight 

 above the dew-point ; while the wetness of their undersides, 

 which are invisible,, brings these experiments into harmony 

 with the experiments with the trays. 



In the second last paragraph of Mr. Tomlinson's paper he 

 seems to object to that part of the " new theory " which relates 

 to the excretion of liquid by plants, principally because it is 

 a startling " and not in keeping with preconceived opinions, 

 and he asks, " for what purpose are plants endowed with such 

 high radiating powers?" I scarcely think he can expect me 

 to answer that question, as it forms no part of my theory. If 

 any one, surely he is the proper person to give the reply. 

 But are plants really such good radiators ? Take even the 

 figures given by Mr. Tomlinson for the different substances 

 named. In none of these is the difference of any importance; 

 indeed the differences are not more than might easily arise 

 from errors of observation by the method used by Melloni, 

 except in the case of vegetable mould. So far as my own 

 measurements go, by the method used by me, I have not 

 found any difference in the radiating powers of grass and soil 

 at night. If there is any difference, it must be very small. 

 One result of recent investigations is to show that plants with 

 leaves like those of grass are very badly adapted for collecting 

 dew, the blades being little narrow strips placed at right angles 

 to the direction of the air-currents ; the slightest movement of 

 the air prevents the surfaces of the blades being cooled by 

 radiation much below the temperature of the air. In illus- 

 tration of this I may mention that narrow strips of glass or 

 other material exposed at night will sometimes collect no 

 dew, while a. sheet of the same material will be running with 

 water, the windward edges being of course dry. Plants with 

 large leaves have quite an advantage over grass in dew-collect- 

 ing powers ; but, on the other hand, grass has the advantage 

 during sunny days, on account of its narrow blades being but 

 little heated by the sun. This may be one reason, though not 

 the only one, why grass so seldom flags under a scorching 

 heat. 



If I might be allowed to make a slight explanation here, 



P2 



