Polytechnic Association. 821 



of course maximum at, or very nearly at, the middle of the stroke ; 

 since it is in the nature of uniform crank motion that the piston 

 velocity should there be greatest. Supposing that, from this point 

 onward, the pressure on the piston should wholly cease, it would 

 nevertheless be true that all this living force would be expended on 

 the crank during the succeeding quarter-revolution ; so that, in case 

 the steam pressure, during the first half-stroke, had been adequate to 

 the production in the same moving mass of twice as much living 

 force, and that one-half of this pressure had gone to the performance 

 of work through the engine, the action upon the crank in the two 

 successive quarter-revolutions would be exactly equal. At first 

 thought it might seem that it should also be symmetrically distri- 

 buted over these two quadrants; but this we shall find not to be the 

 case. 



In order, however, that large force should be thus imparted to the 

 mass of the piston and the moving parts connected with it, it is 

 necessary that these should not only be massive, but that they should 

 move with high velocity. High velocity is therefore a condition as 

 essential to the theory of the Allen engine as large mass in the parts 

 which immediately receive the action of the steam. Now, it has 

 .always been held that the reciprocating parts of a steam-engine 

 should be as light as is consistent with strength ; the force expended 

 in moving these alternately in opposite directions having been 

 regarded as so much thrown away. And in this view of the subject 

 high velocities are objectionable for precisely the same reasqn. Nor, 

 supposing steam to be worked with full head, from beginning to end 

 of the stroke, is this view wanting in plausibility ; but even in that 

 case it is by no means just. The effect under such circumstances 

 would be, not to occasion waste of force, but irregularity of action ; 

 and this irregularity, as will be presently illustrated, would be just 

 the reverse of that resulting from light reciprocating masses and 

 short cut-off. For as, in the latter case, a forward strain on the 

 shaft between the crank and the fly-wheel would occur in the first 

 quarter revolution, and a back strain in the second quarter ; so, in 

 the former, it would be in the second quarter revolution that the 

 forward strain would occur, while the backward would take place in 

 the first. 



Mr. Porter, the gentleman above referred to, as one of the exhibitors 

 of the engine under consideration, has published* a theory of the 



* Engineering ana Mining Journal for January 81, 1871 ; also in Porter'e Description of Richards' 

 •earn Engine Indicator, Lond«, 1866. 



