6 Transactions. 



the ground at night, as Ijodies near the surface get dewed sooner 

 than those higher up, thougli equally cold with them, but con- 

 tended that only a very small part could be derived from this 

 source. Taking up the subject at this point, Mr Aitken instituted 

 a series of experiments, the result of which led him to a different 

 conclusion as to the source of the vapour which condenses on the 

 surface of bodies on dewy nights. Being aware, from previous 

 observations, that the temperature of the soil after sunset at a small 

 depth below the surface was uniformly higher than that of the air 

 above it, and sometimes to the extent of 8° or lO**, he inferred 

 that while this condition existed there would be a tendency for 

 vapour to rise, and pass from the ground into the air, and that, 

 mingling with the air, its moisture would be condensed into dew, 

 wherever it came into contact with a surface cooled by radiation 

 below the dew-point. It then occurred to him that, if vapour were 

 really rising from the ground during the night, it might be trapped 

 on its passage to the air, and that this might be accomplished by 

 placing over the soil shallow pans of tin plate in an inverted posi- 

 tion. The first experiments were made on grass land, and the 

 result is thus stated by Mr Aitken in his paper : — (1) There was 

 always a deposit of dew inside the traps ; (2) There was always 

 more moisture on the grass inside the traps than there vras on the 

 grass outside ; (3) There was sometimes a deposit inside when 

 there was none outside. Experiments of the same kind were made 

 on bare soil, and with the same result. Another method of testing 

 the point under investigation was by cutting out a thin portion of 

 turf from the lawn, and placing it in a small shallow pan six inches 

 square and a quarter of an inch in depth. This portion of turf was 

 then weighed in a fine balance, turned by less than a grain, and 

 restored to the position from which it has been cut out, with the 

 surface of the turf exposed. Having been left in this position for 

 some hours while dew was forming, it was taken up and weighed 

 ao-ain, when it was found to have lost weight to the extent of 24 

 "•rains. This could be explained only by a certain amount of the 

 moisture it contained passing out of it in the form of vapour, part 

 of which would be condensed into dew on the blades of grass, and 

 part escape into the air. Experiments were also made on hard 

 roads and gravel. It is a well-known fact that dew rarely ever 

 appears on the surface of these, even on the most dewy nights. 

 This has often been attributed to the greater radiating power of 



