THE METEOROLOGY. AND CLIMATE OF PLYMOUTH. 431 
heat is abstracted from the substance on which evaporation takes 
place to produce the vapour; hence in general the wet bulb 
registers a lower temperature than the dry. The dew point is that 
temperature at which dew begins to be formed. The formation of 
dew may be seen by any one bringing a glass of cold water into a 
warm room in which many people have been sitting for some time. 
The glass gets dim almost immediately, and moisture forms on the 
outside of the glass. The ratio of the amount of aqueous vapour 
actually present in the atmosphere to that which it would contain 
if it were saturated, is called the hygrometric state or degree of 
saturation. In Glaisher’s tables perfect saturation is placed at 100, 
and according as the air is near or far from saturation, so his 
numbers approach to or recede from 100. The degree of saturation 
does not depend on the amount of aqueous vapour present in the 
atmosphere, but on the nearness of the vapour-laden air to its 
point of saturation. Thus cold air, with but little vapour in it, 
may be moist; whilst warm air, containing much more vapour, 
may be dry. When the wet and dry bulbs act exactly alike it 
shows the air is fully saturated, and the least fall in temperature 
will produce rain, because cold air cannot sustain as much water 
in the form of vapour as warm air can. Thus air at 75° F, 
contains 8 grains of vapour per cubic foot when fully saturated, 
whilst at 32° F. it contains only 2:1 grains. 
Another property of air is worth noticing. It may be shown 
thus: If we mix equal quantities of air fully saturated at different 
temperatures, some part of the vapour will be precipitated in the 
form of water. ‘Take, for example, a cubic foot of air at 70° F., 
and mix with a cubic foot at 32° F., both being fully saturated ; 
the result is, 2 cubic feet at a temperature of their arithmetic 
mean; 7.e. at 51° F. But in the process we shall also have mixed 
8 grains+ 2°] grains of vapour as well. ‘To an ordinary observer 
it would seem no other effect would follow; but the property of 
air I alluded to above is that air at 51° F. can contain only 4:2 
grains of aqueous vapour in a cubic foot; and hence our two 
cubic feet can contain only 8°4 grains of vapour. We have thus 
10:1—8°4=1°7 grains more vapour of water than the air can hold, 
and hence it must be precipitated, and so we get rain. 
What I have now shown on a small scale applies equally to 
large bodies of air; and if such temperatures as I have taken 
should actually occur, we get 83 grains of water precipitated for 
