Mr Addmson 07i RalURoads. 251 



the: motion of the piston and pistonrrod within the cylinder, can-« 

 not be affected by the load. Let c represent this constant quan-' 

 tity : then the remainders 484-c and 113-c may be assumed to 

 have to each other the ratio of the pressures, to wliich the mo." 

 ving parts of the machinery, exclusive of the pistons, have been: 

 subjected. When the engine was unloaded, tliis pressure could) 

 arise only from ^the resistance of the piston. Now, if / repre-; 

 sent the length of the stroke, and d the diameter of the wheel,; 

 then, the constant resistance c will be to the pressure upon the 

 piston, which would counteract it, in the ratio of 1 : . g-i*ig <^ ^: 

 which is the ratio of the spaces passed over by the piston and 

 the engine. When the engine is loaded and working with a pres- 

 sure of 50 lb. per square inch of the pistons of two nine-inch 

 cylinders, the whole pressure on the pistons will be 6367 lb.- 

 which, when diminished in the ratio of 1 : ^•'j-^.xesd ^-^ -^^ ^^ie' 

 pressure producing friction in the other parts of the engine; if J=3 

 and /= 2, its numerical value will be 2702 lb. Part of this is* 

 absorbed by the constant resistance c; and, therefore, 2702-c will- 

 represent the effective load or pressure producing resistance in* 

 the rubbing parts of the engine when loaded. Hence, as the re-' 

 sistances are in the ratio of the pressures, we have 484-c : 113-c 

 r- 2702-c : c and c = 98.3 nearly ; therefore the resistances from 

 friction when the engine was loaded will be 385.7 and when un- 

 loaded 14.7. 



The steam pressure required to overcome the friction of the- 

 pistons in the cylinders will be therefore, 98.3 x ^-^^^ ^ ^ — 231 - 

 lb. This result is remarkable, as it is very far below the theo- 

 retical value of this kind of resistance : since there will be proi 

 bably about 100 square inches of rubbing surface in each cylin- 

 der, the resistance is not quite 1 J lb. per square inch of rubbing 

 surface *. - .. 



The resistance created by the friction of the whole machi- 

 nery, may be expressed as a multiple of either the pressure o^ 

 the steam on the piston, or of the load attached to the engine; 

 and if the numerical values of the quantities in question were tq 



--;* |tt would be interesting to know Avhat pressure will render oiled hempi 

 or such substances as are used in packing the piston, impervious to steam. Th* 

 experiment could be easily made, by exposing the substances (com2)ressed be- 

 tween drilled plates) to the pressure of steam of different elasticities. 



