

S/R WILLIAM SIEMENS, F.R.S. 129 



then the top and bottom rocks formed an element which he 

 thought no calculation could reach. If the working was carried 

 out in a mass of coal of infinite extent, then one might be 

 disposed to consider the resistance to the circumference of the 

 semicircular or shallow cylinder and the sphere which would have 

 to be separated if the force were applied in a mass ; but inasmuch 

 as the layer was confined between sides of rock, and that rock was 

 pressed by a superincumbent weight of untold amount, the 

 question was incapable of solution. On general grounds he 

 believed it would be impossible for the Institution to arrive at 

 any definite conclusion with regard to the best machine. The 

 machines of Mr. Chubb and of Mr. Jones were similar in 

 principle, and were both no doubt powerful though limited in 

 range of expansion ; but, while such limited range was sufficient 

 to separate the coal, he believed that the largest quantity of coal 

 could be got with a minimum expenditure of force, because all 

 the force of that short range was directly applied. It was, how- 

 ever, necessary that the coal should be sufficiently hard to allow of 

 a perfectly cylindrical hole being drilled or bored, and that it 

 should not yield more than the length of the stroke of the pistons. 

 Mr. Bidder's machine, on the other hand, was capable of greater 

 range, and by it greater force could be brought to bear upon one 

 point. It seemed also to have the advantage that a smaller hole 

 sufficed for its action ; but, on the other hand, that hole must be 

 deeper than was necessary in the case of Mr. Chubb's machine. 

 He subscribed to the objection urged by Mr. Mallet, that a pro- 

 portion, not easily ascertained, of the force expended must be lost 

 by pressing the front of the wedge against the mass of dead coal. 

 The wedges pressed not against the coal that was going to be 

 loosened, but against the dead mass of coal further on ; and it was 

 a question, which would puzzle a mathematician to determine, 

 what proportion of force was expended in compressing the coal, 

 and what proportion in cracking it away from its position. With 

 Mr. Bidder's machine the loss of a portion of the pressure by the 

 compression of the coal suggested to Mr. Siemens a modification 

 which was this : where coal was naturally tender, and where it 

 was advisable to work it in comparatively thin layers, there might 

 be applied a machine composed of three strong steel bars, arranged 

 like a pair of scissors, with a hydraulic ram driving at the ends 

 VOL. i. K 



