All About Engines

Fuel and Its Problems 327 leaf, like that of a fern. Others are shaped like branches of trees, while forms like huge tree stumps are found standing vertically in the seams. These and other facts show that it has a vegetable origin— that it is, in fact, composed almost entirely of trees and plants that flourished thousands of years ago, before the vast overlying rocks had been deposited upon them. Of the actual structure—leaf and fruit and stem and branch—few traces remain, and coal is, for the most part, formless. It is not crystalline ; it shows no evidence of having been melted or even partially burnt. By some process, the nature of which we can only conjecture, the plants of the car- boniferous age have been changed from what is in many respects an unsatisfactory fuel into one more compact, and containing less moisture and ash. Since plants grow only under the influence of the light and heat of the sun, the world’s coalfields con- tain the stored-up energy of the sun’s activities thousands or tens of thousands of years ago. In the engines and boilers of to-day we are using again some of the energy which, in the form of light and heat, enabled the plants of long ago to store up carbon, which is the principal constituent of all forms of fuel. Coal has, therefore, been called, picturesquely, preserved sunshine, and the term is not inappro- priate because by suitable means water can be boiled and steam produced by the direct rays of the sun. The process of coal formation, however, stopped long ago. Nowhere, in no part of the earth where the explorer has yet trod or the prospector searched,