of Edinburgh, Session 1885 - 86 . 
447 
weight. It was thus evident that vapour was rising from the 
ground while dew was forming, and therefore the dew found on 
the grass was formed of part of the rising vapour, trapped or held 
hack by coming into contact with the cold blades of grass. 
Another method employed for determining whether the conditions 
found in nature were favourable for dew rising from the ground on 
dewy nights, was by observations of the temperatures indicated by 
two thermometers, one placed on the surface of the grass, and the 
other under the surface, amongst the stems, but on the top of the 
soil. The difference in the readings of these two thermometers on 
dewy nights was found to be very considerable. Ten degrees was 
frequently observed, and occasionally it was as much as eighteen 
degrees. A minimum thermometer placed on, and another under, 
the grass, showed that during the whole night a considerable 
difference was always maintained. As a result of this difference of 
temperature it is evident that vapour will rise from the hotter soil 
underneath into the colder air above, and some of it will be trapped 
by coming into contact with the cold grass. 
While the experiments were being conducted on grass land, parallel 
observations were made on bare soil. Over soil the inverted trays 
collected more dew inside them than those over grass. A small area 
of soil was spread over a shallow pan, and after being weighed was 
exposed at the place where the soil had been taken out, to see if bare 
soil as well as grass lost weight during dewy nights. The result 
was that on all nights on which the tests were made the soil lost 
weight, and lost very nearly the same amount as the grass land. 
Another method employed of testing whether vapour is rising 
from bare soil, or is being condensed upon it, consists in placing 
on the soil, and in good contact with it, small pieces of black 
mirror, or any substance having a surface that shows dewing easily. 
In this way small areas of the surface of the earth are converted into 
hygroscopes, and these test-surfaces tell us whether the ground 
where they are placed is cooled to the dew-point or not. So long 
as they remain clear and undewed, the surface of the soil is hotter 
than the dew-point, and vapour is being given off, while if they get 
dewed, the soil will also be condensing vapour. On all nights 
observed, these test-surfaces kept undewed, and showed the soil to 
be always giving off vapour. 
