April i, 1890.] THE TROPICAL AGRICULTURIST. 689 
HOW EAIN IS FOEMED.* 
In certain villages in the Iodiau Central Provtuces, 
besides the village blacksmith, the village accountant, 
the village watchman, and the like, there is an official 
termed the yapoyagri, whose duty it is to make rain. 
So long as the seasons are good and the rain comes in 
due season, his office is no doubt a pleasant and lucra- 
tive one. It is not very laborious, and it is obviously 
the interest of all to keep him in good humour. But 
if, as sometimes happens, the hot dry weather April 
and May is prolonged thiough June and July, and 
week after week the ryot sees his young sprouting 
crops withering beneath' the pitiless hot winds, public 
feeling is want to be roused against the peccant rain- 
maker, and he is led forth and periodically beaten 
until he mends his ways and brings down the much- 
needed showers. 
You will hardly expect me, and I certainly cannot 
pretend to impart to you the trade-secrets of the pro- 
fessional rain-maker. Like some other branches of 
ocoult knowledge which Madam Blavateky assures us 
are indigenous to India, this art of rain-making is per- 
haps not to be acquired by those who have been 
trained in European ideas ; but we can at least watch 
and interrogate Nature, and learn something of her 
method of achieving the same end ; and if her scale 
of operations is too large for our successful imitation, 
we shall find that not only is there much in it that 
may well challenge our interest, but it may enable us 
to some extent to exercise prevision of its results. 
Stated in the most general terms, Nature's process 
of rain-making is extremely simple. We have its aua- 
logue in the working of the common still. First, 
we have steam or water vapour produced by heating 
and evaporating the water in the boiler; then the 
transfer of this vapour to a cooler ; and finally we 
have it condensed by cooling, and reconverted into 
water. Heat is communicated to the water to convert 
it into vapour, and when that heat is withdrawn from 
it, the vapour returns to its original liquid 6tate. 
Nature performs exactly the same process. 
In the still, the water is heated until it boils; but 
this is not essential, for evnporation may take place 
at all temperatures, even from ice. A common little 
piece of apparatus, often to be seen in the window 
of the philosophical instrument maker, and known 
as Wollastou's cryophorus, is a still that works with- 
out any fire. It consists of a large glass tube with 
a bulb at each end, one of which is partly filled with 
water; and, all the air having been driven out of the 
tube by boiling the water, it is hermetically sealed 
and allowed to cool. It then coutains nothing but 
water and water vapour, the greater part of which re- 
condenses when it cools. Now, when thus cold, if the 
empty bulb be surrounded by ice, or, better, a mixture 
of ice and salt, the water slowly distils over, and is 
condensed in the colder bulb, and this without any 
heat being applied to that which originally contained 
the water. And this shows us that all that is ne- 
cessary to distillation is that the condenser be kept 
cooler than the evaporator. 
Nevertheless, at whatever temperature it evaporates, 
water requires heat, and a large quantity of heat, 
merely to convert it into vapour; and this is the case 
with the cryophorus, for if the evaporating bulb be 
wrapped round with flannel, and so protected from 
sources of heat around, the water cools down uutil it 
freezes. That is to say, it gives up its own heat to 
form vapour. A simple experiment that anyone may 
try with a common thermometer affords another illus- 
tration of the same fact. If a thermometer bulb be 
covered with a pieoe of muslin, and dipped into water 
that has been standing long enough to have the same 
temperature as the air, it gives the same reading in 
the water as in the air. But if when thus wetted it 
be lifted out and exposed to the air, it begins to siuk 
at once, owing to the evaporation of the water from 
tb.3 wet surface, and it sinks the lower the faster it 
dries. In India, when a hot wind is blowing, the wet bulb 
sometimes binks 40° below the temperature of the air. 
* A Leoture delivered by H. F. Blanford, F, r. s., at 
the Hythe School of Musketry. 
87 
Now this is a very important fact in connection with 
the formation of rain, because it is owing to the fact 
that water vapour has absorbed a large quantity of 
heat — which is not sensible as heat, but must be taken 
away from it before it can be condensed and return to 
the liquid Btate — that vapour can be transported as such 
by the winds for thousands of miles, to be condensed 
as rain at some distant part of the earth's surface. 
I have said that the quantity of absorbed heat is very 
large. It varies with the temperature of the water that 
is evaporating, and is the greater the 1 >wer that tem- 
perature. From water that is on the point of freezing 
it is such that one grain of water absorbs in evaporating 
as much heat as would raise nearly 5J grains from the 
freezing to the boiling point. This is called the latent 
heat of water vapour. As I have said, it is quite insen- 
sible. The vapour is no warmer than the water that 
produced it, and this enormous quantity of heat has 
been employed simply in pulling the molecules of 
water asunder and setting them free in the form of 
vapour, which is merely water in the 'state of gas. All 
liquids absorb lateut heat when they evaporate, but 
do other known liquid requires so much as water. 
Many things familiar in everyone's experience find 
their explanation in this absorption of latent heat. 
For instance, we feel colder with a wet skin than 
with a dry one, and wet clothes are a fruitful source 
of chills when the body is in repose; although, so 
long as it is in active exercise and producing a large 
amount of heat, since the evaporation only carrier off 
the excess, no ill consequence may ensue. Again, if 
a kettle be filled with ice-cold water aod put on a 
gas stove, suppose it takes ten minutes to bring it 
to boil. In that ten minutes the water has absorbed 
as much heat as raises it from 32° to 212°, an increase 
of 180°. Now if it be left boiling, the gas-flame 
being kept up at the same intensity, we may assume 
that in every succeeding ten minutes the same quantity 
of heat is being absorbed by the water. But it gets 
no hotter: it gradually boils away. And it takes 
nearly an hour, or much than five times as long as 
it took to heat it, before the whole of the water 
has boiled away, since all this heat has been used 
up in coverting it into steam. It was by an experi- 
ment of this kind that Dr. Black, in the last century 
discovered the fact of latent heat, and determined its 
amount ; and it was the knowledge of this fact that 
led James Watt to his first great improvement in 
the steam-engine. 
One more example I may give, which those who 
have been in India will be able to appreciate, and 
which those who intend to go there may some day find 
useful to know. Nothing is more grateful in hot dry 
weather than a drink of cold water. Now ice is not 
always to be had, but when a hot wind is blowing, 
nothing is easier than to gat cold water, if you have a 
pot or bottle of unglazed earthenware, such as are to be 
had in every bazaar, or, what is better, a leather 
water-bottle, called a chhayal, or a water-skin. All 
these allow the water to soak through and keep the 
outside wet ; and if any one of th ;m be filled with 
water an! hung up in a hot wind, in the course of 
half an hour or an hour, the evaporation from the out- 
side will have taken away so much heat that the con. 
tents may be cooled 20° or 30°, notwithstanding that 
the thermometer may stand at 110 Q or 115° in the 
shade. Soda-water may be ccoled in the same way if 
wrapped in straw and kept well wetted while exposed 
to the wind. But it is of little use to do as I have 
seen natives do sometimes, viz., put the bottles into a 
tub of water in a closed room. It is the evaporation 
that carries off the heat, otherwise the water is no 
cooler than the air around. 
Now to return to our subject. The atmosphere 
always contains some vapour which the winds have 
taken up from the ocean, lakes, rivers, and even from 
land, for there are but few regions so dry and devoid 
of vegetation that there is no moisture to evaporate. 
The quantity of water thus evaporated from large 
water surfaces is a question of some importance to 
engineers, who have to take account of the loss from 
reservoirs, and irrigation tanks, and a good deal of 
attention has been given to measure the amount 
