276 OF THE APPLICATION [SECT. ix. 



a view to easy repair. When coals are not expensive, the most simple methods 

 are the most economical ; for instance, at the mouth of a coal pit, the extra con- 

 sumption of coals is of less value than the extra wear and tear of a complex 

 engine. 



574. The modes of draining mines are dependent on the nature of the district 

 where they are situated. If it be mountainous, a subterraneous channel or day 

 level drift 1 may be made, from the lowest part of the mine to terminate in the 

 nearest valley, to carry off the water ; and it is only when this method is imprac- 

 ticable that water is raised by power, and even then the water is raised no higher 

 than to where a day level drift can be obtained. But it frequently happens that 

 the flatness of the country renders any other method impracticable, than that of 

 raising the water to the surface. For example, in the coal-field of Northumberland 

 and Durham, many of the large double pits exceed one hundred fathoms in depth, 

 and some are nearly 150 fathoms deep, with no means of drainage by levels. 

 These pits therefore require very powerful engines, and lately they have chiefly 

 erected double engines, some of which are above 100 horse power : the largest 

 I saw there was one on the south side of the Tyne, which was working with 

 160 horse power, and was capable of exerting the power of 200 horses in action 

 at once. In Cornwall they have some larger engines ; but two engines should 

 always be preferred, when the cylinder of one engine would exceed about sixty 

 inches in diameter, for two engines give many advantages. 



575. When double engines are used for lifting water, they generally work 

 one set of pumps by the outward end of the beam, and another set by a diagonal 

 spear from the piston rod end. And in cases where it has not been convenient 

 to divide the pumps into two sets, the ascending motion of the piston has been 

 employed to raise a weight equal to the pressure of half the column of water in 

 the pumps ; but for such cases a single engine should be preferred. 



576. The following table (p. 278.) will give some idea of the work done by a 

 given quantity of fuel, and of the nature of the engines most approved of in Corn- 

 wall ; the results however can be correct only through the different errors of the 

 mode of estimation balancing one another, for the weight of the column of water 

 is less than the resistance, and the counter only registers the strokes, and not the 

 actual quantity of water raised. 8 



1 In Cornwall and Devon it is called an adit, in some other places a sough. 



* In the year 1811, a number of the respectable proprietors of the valuable tin and copper mines 

 in Cornwall resolved that the work which their respective steam engines were performing, should 

 be ascertained, as it was suspected that some of them might not be doing duty adequate to the 

 consumption of fuel; and for the greater certainty of attaining their object, it was agreed that a 

 counter should be attached to each engine, (art. 563.) and all the engines be put under the super- 



