This cut is intended to represent what may be a section of the subterranean reservoir,S W, of the great Geyser, and the pipe,P, connected with the saucer-shaped basin,B, at the top of the mound. You see the cracks in the rock through which you must suppose hot water constantly trickling into the reservoir.
The spaceSwill be constantly full of steam, and the space below,W, will be always full of water. The water will continue to rise till it gets toA, and it will then quite stop up the escape of the steam.
The steam inSwill then press upon the surface of the water, and force it up the pipePwith more or less violence, according to the supply of heat; but when enough of the steam has escaped to render the pressure less, and the water has sunk to nearI, the eruption will cease.
If you will take the trouble to understand this section, you will see how reasonable it is that the Geyser should be so constructed; and I have seen a little apparatus made of glass, which showed exactly the same sort of operations.
Some little variation in the shape of the reservoir and pipe, may perhaps be needed to account for all that takes place; but the principle of the activity being produced by the agency of steam, acting in a space which may be enclosed by the water in its rising to a certain height, seems to be certain and satisfactory.