A FEW WORDS ABOUT COAL. 307 



it is satisfactory to have at length obtained such posi- 

 tive proofs of showers of rain, the drops of which 

 resembled in their average size those which now fall 

 from the clouds. From such data, we may presume 

 that the atmosphere. of the carboniferous period corre- 

 sponded in density with that now investing the globe, 

 and that different currents of air varied then as now in 

 temperature, so as to give rise, by their mixture, to the 

 condensation of aqueous vapour.' 



If we now turn to the consideration of the extent of 

 the earth's surface occupied by those particular strata 

 which belong to the coal period, we find evidence of 

 the existence of enormous quantities of available coal. 

 Professor Ansted mentions that a quarter of a million 

 of square miles of the earth's surface ' are covered with 

 sandstones and shales of the carboniferous period 

 among which coal is buried ; and this coal is for the 

 most' part accessible.' Now there are upwards of three 

 million square yards of surface in a square mile ; and, 

 assuming an average total thickness of ten yards for 

 all the distinct seams of each coal-field, we find for the 

 total number of cubic yards of available coal the enor- 

 mous figure 7,500,000,000,000. As a cubic yard of coal 

 weighs nearly a ton, we may say that there are in round 

 numbers seven billions of tons of coal available for the 

 use of the human race. If we took the average number 

 of human beings living at each moment during the next 

 3,500 years to be 2,000 millions, and the annual con- 

 sumption for all purposes to be at the average rate of 



x 2 



