16 MR JOHN AITKEN ON DEW. 



lost in the manipulations ; after which it was again restored to its place in 

 the lawn, and left exposed while dew was forming. A few experiments were 

 made in this way, in all of which the ground was found to lose weight. For 

 instance, on the 7th October, the small turf freely exposed to the sky at 

 5.15 p.m., when weighed again at 6.30 p.m., was found to have lost 5^ grains 

 (0-356 grammes), and by 10.15 p.m. it had lost 24 grains (1*555 grammes). 

 Fuller particulars of these experiments will be given further on. 



In making these experiments, the first thing done was to sink two thermo- 

 meters in the ground, one to a depth of 3 inches (76 mm.), the other to a depth 

 of 1 foot (305 mm.), and to place a third thermometer on the surface of the grass. 

 Readings were taken when the experiment began, and again when the pans 

 were removed for weighing. During the time the turfs were exposed, generally 

 about 5 hours, the soil at 3 inches below the surface lost from 2 to 5 degrees, 

 while at 12 inches the loss was small. No doubt part of the heat was lost by 

 radiation, but in grass-land, where the surface of the soil is protected by a fairly 

 good non-conductor, much of the heat will be spent in evaporating the moisture. 



These experiments prove clearly that under the conditions then existing, 

 the soil loses weight, and that vapour really rises from the ground even while 

 dew is forming ; therefore the dew then found on the grass must have 

 been formed out of the vapour rising from the ground at the time. The dew 

 on the grass was, in fact, so much of the rising vapour trapped by the cold 

 grass. The blades of grass acted as a kind of condenser, and held back some 

 of the vapour which would have escaped into the air. 



It must not be supposed that these experiments in any way contradict the 

 well-known observations of Wells and others who have worked at the subject. 

 It lias long been the custom to expose different substances to radiation during 

 the night, and to estimate the amount of dew on different nights by the increase 

 of weight due to the moisture collected on them. It must be noticed that the 

 conditions of the two sets of experiments are quite different. In those for 

 estimating or measuring the amount of dew, the collecting body must not be 

 in heat communication with the earth, an essential condition being that it shall 

 receive no heat by conduction from surrounding bodies ; whereas, in the experi- 

 ments with the turf, the essential condition is that the body experimented on 

 shall be in as good contact with the ground as possible. The result of these 

 two conditions is, that in the former, the exposed surface loses heat by radiation 

 into space, and soon gets cooled below the temperature of the air, and when 

 cooled below the dew-point, dew collects upon it ; while in the latter case the 

 exposed surface is in good heat communication with the ground, and tends to 

 keep hotter than the other surface ; then being always moist it tends to give off 

 vapour, which diffuses away from the hot ground and escapes into the air 

 above, but in part is trapped by coming into contact with the cold grass. 



