This section is from the book "Health", by W. H. Coefield.
When one mass of air containing a large quantity of water dissolved in it meets another of the same bulk which is colder than itself, and they mix, the resulting mixture has the mean temperature of the two masses mixed. Suppose that a mass of air at 32° Fahr., the freezing point of water, meets an equal mass of air at 96°, both being saturated with water, the mass of air at 32° will contain about 2 grains of water, and the mass at 96° about 17£ grains of water in each cubic foot. When these two mix together they form a mass of air having a temperature which is the mean of the two temperatures, namely, 64°. Will that mass of air hold all the water in solution that the other two held? At first sight you might think that it would, but you must remember I told you that air at 64° would only hold 6£ grains per cubic foot, so that when these two masses of air at 32° and 96° mix together, they form a mass at 64°, which will only hold 6£ graihs per cubic foot, instead of 9| grains, so that the difference of 3 J grains per cubic foot can no longer be dissolved by the air and it falls; that is the way rain is produced.
The amount of rain that falls varies veiy much in different countries. Hot air dissolves much more water than cold air, and so the hottest air in the world is likely to have much more water in it than colder air, so the hottest place is likely to have much more rain ; thus the greatest amount falls at the equator, and the amount decreases as we go to the poles.
The amount of rain that falls in a place is measured in England in inches. You will see in the reports that are published in the newspapers that an inch of rain has fallen in a certain time; what does that mean ? It means that during the time specified an amount of rain has fallen over that particular place, which, if none of it had evaporated or been soaked up by the ground, would cover that place to the depth of an inch. The amount of rain is measured by an instrument which goes by the name of the rain-gauge. It is a kind of funnel, upon which the rain is allowed to fall; the area of the funnel is known, and it can then be calculated accurately how much rain has fallen in a given time. The chief protection necessary is that the rim of the funnel shall be vertical, so that the drops that fall into it shall not splash out. Let us suppose the area of the funnel to be 106 square inches; when an inch depth of rain falls upon 100 square inches, there will be 100 cubic inches of water; so when 100 cubic inches of water have collected in the vessel below the funnel, one inch of rain has fallen.
The amount of rain that falls in London is from 22 to 23 inches per year on an average. Much more falls in some parts of England, on the west coast of England generally, and in Cumberland as much as 150 inches have fallen. In other countries as much as 400 or 450 inches, and in one place in India 600 inches, fall in a year.
No connection has been made out between the amount of rain that has fallen and the number of deaths in a place, but one thing is certain, and that is, that the rain clears the air. In falling it drags down with it, or dissolves, impurities, and so a heavy rainfall tends to purify the air. Another thing that it does is, it flushes the sewers, and so clears out a large amount of impurity out of the drains as well as out of the air. And it has been repeatedly observed that epidemic diseases have disappeared sometimes quite suddenly after very heavy rainfalls. I have always considered it a great advantage in the summer during the prevalence of epidemics of typhoid fever, if we have one or two heavy thunder storms. And it has been noticed over and over again that the mortality from such diseases has been less after a heavy rainfalL
Of the rain that falls upon the ground, part is evaporated, part is absorbed by plants and subsequently evaporated, part runs off the surface, and part penetrates into the soil; the water penetrates through the soil as long as the soil is porous, until it comes to an impervious stratum, along the surface of which it runs until that impervious stratum comes to the surface of the ground, and there the water comes out and forms what we call a spring. The water penetrating the soil dissolves certain matters out of it, and so spring waters are generally very hard waters; sometimes so hard as to be petrifying springs, generally cold, but sometimes in volcanic countries, for one reason or another, they become hot, and may be actually boiling. Springs run together and form rivers, and in so doing a large quantity of carbonic acid, which spring waters contain, is given out into the air, and a good deal of carbonate of lime is deposited, so river waters are generally softer waters than spring waters, but, in other ways, more impure on account of the impure matters thrown into rivers. Rivers affect the climate of places; thus in their neighbourhood it is cooler in summer than away from them, and warmer in winter until the river freezes. They also affect the climate by evaporation from their surface, and so the neighbourhood of rivers is a moist neighbourhood, and the effluvia which rise from them, especially when a large quantity of impure matter has been thrown into them, are often deleterious to health ; but I must tell you that the exact effect of the effluvia given out has not been determined (although they must have an effect), because in all the instances there are so many other causes at work.
Rivers also often influence the climate by inundating the country, and by the large Amount of material that they bring down, which they deposit at various parts of their course or at their mouths, so forming unhealthy swamps.
To give you an idea, I will take rather an extreme case, that of the Ganges, which Sir Charles Lyell tells us deposits 6368 million cubic feet of solid matter in a year, and brings down of that more than 6000 million cubic feet of suspended matters during the rainy months.