Proceedings of the Polytechnic Association. 949 



law of Boyle and Marriotte, or the hyperbolic formula. It should 

 be recollected that all these calculations are on the indicator power, 

 taken without any allowances for friction. The pressure being less 

 than that of the atmosphere, during nearly half the stroke, it 

 requires no explanation to see that all the friction of the piston, 

 the piston rod, &c., during that useless or rather negative portion of the 

 stroke, was a dead loss, and a deduction of a similar kind should be 

 made for friction at every point. The negative effect of the pressure 

 alone, during the last portion of the stroke in very expansive work- 

 ing, is in every instance deducted from the positive efi'ect of the 

 other portions before entering the power in the table. If due allow- 

 ance is made in eacli computation for what some investigators have 

 termed the " unloaded friction " of the engine, which is of course 

 ecpial at all poinl^ in the stroke, and being due to the tightness of the 

 packing, the weiglit of the parts, the diameter of the journals, &c., 

 is equal to that which would obtain if the steam followed at its full 

 pressure through the whole, instead of a portion of the stroke ; if due 

 allowance is made for this element of resistance, and for this peculia- 

 rity thereof, its constancy, it will be seen that the loss in these engines, 

 due to a too high degree of expansion, is in practice very consider- 

 able, except when loaded up to very nearly their maximum power. 



