742 THE POPULAR SCIENCE MONTHLY. 



coal-beds having been at one time a forest, growing under the full 

 power of a brilliant sun, the result of solar forces, produced then, as 

 now, by chemical phenomena taking place in the sun itself. Every 

 cubic yard of coal in every coal-bed is the result of a very slow, but 

 constant, change of a mass of vegetable matter; that change being 

 analogous to the process of rotting in a large heap of succulent plants. 

 The change has been so slow, and continued under a constantly-in- 

 creasing pressure, that but few of the gaseous constituents have es- 

 caped, and nearly all those physical forces which were used in the 

 task of producing the woody matter of the plant have been held 

 prisoners in the vegetable matter which constitutes coal. How vast, 

 then, must be the store of power which is preserved in the coal de- 

 posits of these islands ! 



We are now raising from our coal-pits nearly one hundred and ten 

 millions of tons of coal annually. Of this quantity we are exporting 

 to our colonial possessions and foreign parts about ten million tons, 

 reserving nearly a hundred million tons of coal for our home consump- 

 tion. Not many less than one hundred thousand steam-boilers are in 

 constant use in these islands, producing steam to blow the blast for 

 smelting the iron-ore to urge the mills for rolling, crushing, and cut- 

 ting with giant power to twirl the spindle and to urge the shuttle. 

 For every purpose, from rolling cyclopean masses of metal into form 

 to weaving silky textures of the most filmy fineness, steam is used, and 

 this steam is an exact representative of the coal employed, a large 

 allowance being made for the imperfections of human machinery. This 

 requires a little explanation. Coal is a compound of carbon, hydrogen, 

 oxygen, and nitrogen, the last two elements existing in quantities so 

 small, as compared with the carbon, that they may be rejected from 

 our consideration. The heat which we obtain in burning the coal is 

 almost all derived from the carbon ; the hydrogen in burning produces 

 some heat, but for our purpose it is sufficient to confine attention to the 

 carbon only. 



One pound of pure coal yields, in combining with oxygen in com- 

 bustion, theoretically, an energy equal to the power of lifting 10,80S,- 

 000 pounds one foot high. The quantity of heat necessary to raise a 

 pound of water one degree will raise 772 pounds one foot. A pound 

 of coal burning should yield 14,000 units of heat, or 772 x 14,000=10,- 

 S0S,000 pounds, as above. Such is the theoretical value of a pound of 

 pure coal. Many of our coal-seams are about a yard in thickness ; sev- 

 eral important seams are much thicker than this, and one well-known 

 seam, the thick coal of South Staffordshire, is ten yards in thickness. 

 This, however, concerns us no further than that it is useful in convey- 

 ing to the mind some idea of the enormous reservoir of power which 

 is buried in our coal formations. One square yard of the coal from a 

 yard-thick seam that is, in fact, a cubic yard of coal weighs about 

 2,240 pounds avoirdupois ; the reserved energy in that cube of coal is 



