In the accompanying illustration we have an example of the Condensating principle as applied to the steam-engine, and popularly known as the “Low-pressure Engine.” In this case force is reconverted, so to speak, and, if a cubic inch of water has been converted by heat into a cubic foot of steam, creating a pressure in one direction, it can be reconverted by cold, and so produce a pressure in another direction.
It is owing to this fact that some parts of the world are always hot and always wet, Guiana being a striking example.
The wind blows over the ocean, absorbing moisture as a sponge does water. As it passes from the sea over the land, it is met by secondary mountain ranges, too low to arrest its progress altogether, and high enough to have their summits clothed in eternal snows. As soon, therefore, as the warm, water-laden winds pass over these mountains, the moisture is condensed by their frozen tips, and down rushes the rain in torrents.
Even in our own temperate land we can often trace the cause of a heavy rain to the presence of a lofty hill, or even an exceptionally tall spire. The moist climate of Oxford has been attributed by scientific men quite as much to its spires and towers as to its low-lying situation.
Now we come to the various modes of extracting the water which is laid up within the earth, and which only slowly ascends to the surface when drawn up by the heat of the sun.
Water is everywhere, but the depths at which it is found are vastly different. For example, at one house in which I lived it was not possible to dig for three feet without coming to water. In another, no water was found within some two hundred feet, and, as I several times relieved the old gardener of the task of drawing the water for the day’s consumption, I have reason to remember the depth.
The pail, rope, and winch which were in use at that time—and may be still, to the sorrow of the gardener—are but a sort of semi-savage way of procuring water from the depths of the earth. It is a well-known fact that under certain conditions water always finds its own level, minus the friction of the channel through which it passes. On this principle all fountains are made. Those, for example, at the Crystal Palace, which fling their waters to such a height, are fed from tanks on the summit of the two great water towers. And, were it not for the friction of the water in the tubes, and that of the air, the fountains would rise as high as the tanks from which they are fed.
Such is the case with springs, especially with those of an intermittent character, in which latter instance the rushing of the water is exactly coincident with the filling of the hidden tank which supplies it.