A FEW WORDS ABOUT COAL. 315 



Now, the consumption of coal is at present, and for 

 very obvious reasons, passing through the more rapidly- 

 ascending portion of its wave of \ increase. For many 

 years after the first recognition of the value of coal as 

 fuel the mineral crept slowly into use. With its em- 

 ployment, fresh uses for it were found, the very useful- 

 ness of the mineral suggesting new wants. Chief 

 among the results which sprang directly from the use 

 of coal as fuel, was the application of the steam-engine 

 to a number of purposes which had before been either 

 unthought of or unattainable for want of proper fuel. 

 The spread of manufacture, of trade, of travel, and 

 general intercommunication, followed in due course, at 

 once directly and indirectly necessitating a continued 

 increase in the quantity of coal employed throughout 

 the kingdom. These causes are still in full operation ; 

 and it is to be expected that, while this is the case, 

 there will not merely be a steady annual increase in 

 the use of coal (for such an increase would follow from 



readily understand what is here meant by wave-like progression, and 

 obtain also very convincing evidence of the fact in question. Along a 

 horizontal line let equal spaces be measured, and let a set of vertical 

 lines be pencilled through the divisions on the horizontal line. Now, 

 from the weekly records of health let the number of deaths due to any 

 disease, or form of disease as, for instance, diarrhoea, or the class of 

 diseases included under the head zymotic be noted from the commence- 

 ment to the end of some period in which such diseases may have been 

 particularly active, and let the number of deaths in successive weeks be 

 represented on any convenient scale on the successive vertical lines, 

 measuring upwards from the horizontal line. [For example, say that 

 50 deaths shall be represented by one inch, and other numbers propor- 

 tionately.] Then through the summits of the lines thus drawn let a 

 curve be swept. It will be found that this curve has the wave-figure 

 spoken of above. 



