1856.] Physical Geography of North America. 185 



It is interesting to compare the dynamic force of coal applied 

 as fuel to the generation of steam in the steam-engine, with the 

 dynamic effect of a man. The human labourer, exerting his 

 strength upon a treadmill, can raise his own weight, say 150 lbs., 

 through a height of 10,000 ft. per day, equivalent to 1 lb. raised 

 1,500,000 ft. The mechanical virtue of fuel is best estimated by 

 ascertaining the number of pounds which a given quantity, say one 

 bushel, will raise to a given height, say one foot, against gravity. 

 In the steam-engine this is called the duty of the fuel. Now, the 

 present maximum duty of one bushel of good coal in the improved 

 Cornish steam-engines, is equivalent to 100,000,000 lbs. lifted 

 through one foot ; but one bushel has been made to raise 125,000,000 

 lbs. one foot high, or one pound 125,000,000 of feet ; but as there 

 are 84 lbs. in one bushel, this divisor gives 1 iK)und as equal to 

 1,500,000 ft.; just the result of a man's toil for one day upon a 

 treadmill. Thus a pound of coal is really worth a day's wages. If 

 we estimate a lifetime of hard work at 20 years, giving to each year 

 3(X) working days, we have for a man's total dynamic effort 6000 

 days. In coal this is represented by the amazingly small amount 

 of three tons. Another proof of the extraordinary power derivable 

 through the combustion of fuel, is presented in the following calcu- 

 lation ; one cubic inch of water is convertible into steam, of one 

 atmospheric pressure, by 15|^ grains of coal, and this expansion of 

 the water into steam is capable of raising a weight of one ton the 

 height of a foot. The one cubic inch of water becomes very nearly 

 one cubic foot of steam, or 1728 cubic inches. When a vacuum is 

 produced by the condensation of this steam, a piston of one square 

 inch surface, that may have been lifted 1728 inches, or 144 feet, 

 will fall with a velocity of a heavy body rushing by gravity through 

 one half of the height of the homogeneous atmosphere, or through 

 13,500 feet. This gives a terminal velocity of 1300 feet per second, 

 greater than that of th^ transmission of sound. From this we can 

 form some estimate of the strength of the tempest which alternately 

 blows the piston in its cylinder, when elastic steam of high pressure 

 is employed. Applying the calculations of the dynamic efficiency 

 of coal, for estimating the mechanical strength latent in the coal- 

 fields of the earth, or in the large coal product annually furnished 

 by the mines of Great Britain, we get some interesting results. 

 Each acre of a coal seam, four feet in thickness, and yielding one 

 yard nett of pure fuel, is equivalent to about 5000 tons ; and 

 possesses, therefore, a reserve of mechanical strength in its fuel 

 equal to the life-labour of more than 1600 men. Each square mile 

 of one such single coal bed contains 3,000,000 of tons of fuel ; 

 equivalent to 1,000,000 of men labouring through twenty years of 

 their ripe strength. Assuming, for calculation, that 10,000,000 of 

 tons, out of the present annual product of the British coal mines, 

 namely 65,000,000, are applied to the production of mechanical 

 power, then England annually summons to her aid an army of 



