446 
Proceedings of the Royal Society 
4. On Dew. By Mr John Aitken. 
{Abstract.) 
The first point referred to in this paper is the source of the 
vapour that condenses to form dew. A few observations of the 
temperature of the ground near the surface and of the air over it, 
first raised doubts in the mind of the author as to the correctness of 
the now generally-received opinion that dew is formed of vapour 
existing at the time in the air. These observations, made at night, 
showed the ground at a short distance below the surface to be 
always hotter than the air over it ; and it was thought that so long 
as this excess is sufficient to keep the temperature of the surface of 
the ground above the dew-point of the air, it will, if moist, give off 
vapour ; and it will be this rising vapour that will condense on the 
grass and form dew, and not the vapour that was previously present 
in the air. 
The first question to be determined was whether vapour does, or 
does not, rise from the ground on dewy nights. One method tried 
of testing this point was by placing over the grass, in an inverted 
position, shallow trays made of thin metal and painted. These 
trays were put over the ground to be tested, after sunset, and 
examined at night, and also next morning. It was expected that, 
if vapour was rising from the ground during dewy nights, it would 
be trapped inside the trays. The result in all the experiments was 
that the inside was dewed every night, and the grass inside was 
wetter than that outside. On some nights there was no dew on the 
outside of the trays, and on all nights the inside deposit was heavier 
than the outside one. 
Another method of testing this point was employed, which 
consisted in weighing a small area of the exposed surface of the 
ground, as it was evident that if the soil gave off vapour during a 
dewy night it must lose weight. A small turf about 6 inches 
(152 mm.) square was cut out of the lawn and placed in a small 
shallow pan of about the same size. The pan with its turf, after 
being carefully weighed, was put out on the lawn in the place where 
the turf had been cut. It was exposed for some hours while dew 
was forming, and on these occasions it was always found to lose 
