690 
THE TROPICAL AGRICULTURIST. [Avkil i, 1890, 
lost by evaporation. In England it has been fouud to 
vary iu different years from 17 to 27 inches in the 
year, or say from 1J to inches per month on an 
average. Now, s-iace in the east of England the rain- 
fall is only about 24 inches in the year, it follows 
that in that part of the kingdom the loss by evapora- 
tion from a water surface is not very much less 
than the rain falling directly on the surface- 
In dry countries the evaporation may exceed the 
local rainfall. In the tropics it has been found to 
average from 3| to 6 inches per month in the dry 
season. In the case of a large tank atNagpur, constructed 
to supply the city with water, it was found that the 
loss bj evaporation, in the hottest and driest weather 
was two and a half times as great aa the quantity 
supplied for consumption. 
These statistics will give some idea of the enormous 
evaporation that goes on from the water surfaces of 
tbe globe, and to this must be added all that takes 
place from tbe land. In the case of light showers, 
nearly the whole of the raiu is rt-evaporated ; and 
probably, on an average, half of the total rainfall on 
the land is thus lost sooner or later, leaving not more 
than half for the supply of springs and rivers. 
The quantity of vapour in the air is very variable. 
To us, in England, tae west and south-west winds 
are the dampest, coming direct from the Atlantic, and 
north-east winds are the driest. The cause of their 
extreme dryness I shall endeavour to explain presently. 
It is no doubt partly due to the fact that they reach up 
another cause to which I shall have to advert later on. 
The quantity of vapour in the air is usually ascer- 
tained by the hygrometer, the ordinary form of which is 
a pair of thermometers, one having the bulb wet, the 
other dry, and observing the depression of the wet bulb. 
The principle of this I have already explained. But the 
same thing may be ascertained more directly by passing 
a measured quantity of air through a light apparatus 
containing sulphuric acid, or some other substance that 
absorbs water vapour greedily, and weighing the whole 
before and afterwards. The increase of the second 
weighment gives the weigtt of water absorbed. By 
such means it has been ascertai led that air at 60° can 
contain as much as 5| grains of vapour in each cubic 
foot, and hot air at 80° can contain rather less than 11 
grains in the same space. The quantity that air can 
hold increases therefore very rapidly with the tempera- 
ture. But it is seldom that it contains this maximum 
amount, especially at the higher temperatures. 
In order to condense any part of this vapour we must 
take away its latent heat. It is not sufficient merely to 
cool it till it reaches the temperature of condensation, 
but we have further to abstract 5| times as much heat 
as would raise the condensed water from the freezing to 
the boiling point. Before, however, proceeding to 
consider how this cooling is effected, the question arises, 
What is the condensing point? For, obviously, since 
water can evaporate at all temperatures, so we should 
expect that it may condense at all temperatures. On 
what, then, does the condensing point depend ? 
I mentioned just now that air at the temperature of 
60° can contain as much as 5} grains of vapour, and 
at 80° rather less than 11 grains in each cubic foot. 
Obviously, then, if air at 80°, containing this maximum 
quantity, be cooled to 60°, it must get rid of more than 
6 grains, or nearly half its vapour, and this excess 
must be condensed. I speak of air containing these 
quantities, but in point of fact it makes no appreciable 
difference whether air be present or not, An exhausted 
glass vessel of one cubio foot capacity can hold 5| 
grains of vapour at 60° and no more, and nearly 1 L 
grains at 80° and no more ; and if, when thus charged 
at 80°, its contents be cooled to 60°, more than 5 grains 
will be condensed. If, however, it contain only 5| 
grains at 80°, none will condense until the temperature 
falls to 60°, but any further cooling produces some con- 
densation. Thus, then, the condensing point depends 
on the quantity of vapour present in the air, and is 
the temperature at which this quantiiy is the maxi- 
mum possible for that temperature. 
This preliminary point being explained, we may now 
proceed to inquire what means Nature employs to con« 
denbe the vapour in the air, producing at one time dew 
and hoar-frost, at another time fog and cloud, and at 
another ram, hail, and snow. 
Let us take the case of dew and hoar-frost first, as 
they are comparatively simple. And in connection 
therewith I may relate a little incident that took 
plsce at Calcutta some years ago. A gentleman, who 
had not much acquaintance with physical science, 
was sitting one evening with a glass of iced brandy 
and water before him. It was in the rainy season, 
when the air, 1 hough warm, is very damp, and he 
had a large lump of ice in hiB tumbler. On taking 
it up, he noticed to his surprise that the glass was 
wet on the outside, and was standing in quite a little 
pool of water on the table. At first he thought 
i his tumbler was cracked, but putting his finger to 
his tongue he found the fluid tasteless. " Very 
odd!" he remarked; "the water comes through the 
glass but tbe brandy doesn't." 
Now, however with our present knowledge we may 
be inclined to smile at the simplicity of this remark 
it so happens that up to the end of the last century 
very much the same explanation was popularly held 
to account for dew. It was supposed to be a kind 
of perspiration emitted from the earth, and no satis- 
factory explanation of the phenomenon had been 
arrived at by the physical philosophers of the day. It 
remained for Dr- Wells to prove, by a long series of 
observations and experiments, which have been 
quoted by Sir John Herschel and Mr. John Stewart 
Mill as a typical instance of philosophical inquiry, 
that the cold surface of grass and shrubs condenses 
the vapour previously held in suspension in the i ir, 
these surfaces being cooler than the air, and below 
its point of condensation. And such of course, is 
also the case of the glass tumbler containing ice. 
Any one may try the experiment for himself. To 
produce hoar-frost, it is only necessary to cool the con- 
densing surface below the freezing point, which may 
be done by crushing some ice and mixing it with salt, 
A tin pot is better than a glass to make this experiment. 
When not only the ground, but also the air to a 
considerable height above it, is cooled in like manner, 
we have the production of fog, fog being the form 
in which the vapour is first condensed, and consisting 
of water in drops too minute to be separately visible. 
The formation of fog is very much aided i£ the air be 
laden with smoke. Smoke consists of extremely 
minute particles of unburnt coal or other fuel, and 
these cool faster than the air at night, and so cool 
the air in contact with them. Each one of them, 
too, condenses water on its surface, and being thus 
weighted they sink and form that dense fog that 
Londoners know so well. 
Clouds are essentially the same as fog, but formed 
high up in the air. But in their case, and that of 
rain, snow, and hail, another and different cooling 
agency comes into play, and this will require some 
preliminary explanation. 
I dare say that some of you may at some time or 
other have charged an air-gun, And if so, you will 
be aware that when so charged the reservoir becomes 
pretty warm. Now this heat is produced, not, as 
might be supposed, by the friction of the piston in 
charging, but is due to the fact that work has been 
done upon the air by compressing it into a very 
small space ; in other words, work has been con- 
verted into heat. If the compressed air be allowed 
to escape at once, its heat is re-converted into 
work. It has to make room for itself by thrusting 
aside the atmosphere into which it escapes, and when 
thus expanded it is no warmer than before it was 
compressed. Indeed, not so warm, for it will already 
have parted with some of its heat to the metal 
chamber which contained it. And if when compressed 
it is allowed to cool down to the ordinary tempera- 
ture, and then to escape, it will be cooled below that 
temperature just as much as it waa heated by com- 
Eression. Thus, if in being compressed it had been 
eated 100°, say from 60° to 160°, and then allowed to 
cool to 60°, on escaping it will be cooled 100° below 
60°, or to 40° below zero, which is the temperature 
at which mercury freezes. This is the principle of 
the cold air ohambers now so extensively employed 
