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whenever we please. Sir W. TLompson, tlie great electrician, 
has suggested that " where there was a kind of force which 
could be wound up and let go at any moment, and whenever 
we liked, and be turned into energy, as in the case of a clock, 
it should be called ' Potential Energy.' " JN'ow, of all forms 
of potential energy, the most wonderful is coal. If we take 
one cwt. of coal, and let off all its force suddenly, as when 
we would let off a watch spring, and use this force in raising 
the coal up, it would shoot up more than 2,000 miles high, 
even if the force of gravity acted the whole distance ; or it 
would raise 112,000,000 lbs. of coal one foot high. 
This force is turned into use by means of heat, and this is 
the power used in reducing metals, and for almost every 
chemical change that takes place ; while we have the inven- 
tions and improvements of Watt and Stephens on the steam 
engine, stationary and locomotive, to turn it into mechanical 
motion. 
Every improvement of Mr. Fairlie in the steam engine 
must increase the power of coal ; every invention for con- 
suming smoke and preventing the waste of heat, is a step in 
this direction ; and every cook who husbands the use of coal 
in her kitchen fire, contributes indirectly to the support and 
prolongation of the greatness of the British empire. For 
if water-power and wind-mills cannot compete with coal now, 
what are the chances that they will ever again be able to do 
so, when science is every day making coal more powerful ? 
TVe must in truth now regard water-wheels and wind-mills 
as mechanical inventions of a bygone age. 
The consumption of coal within less than sixty years has 
probably increased sevenfold, while the increase of wealth 
and population has been proportionate. If this consumption 
of coal continues to increase at such a rate, the production 
of coal must keep pace with it. In a few years' time per- 
haps even horse-ploughing, and reaping with scythes and 
