Proceedings of the Polytechnic Association. 945 



larger size tlian usual, to allow as free and rapid ingress and egress 

 for the steam as possible ; and to avoid the evils due to the elasticity 

 of ordinary cords, the motion of the engine was communicated to 

 the cylinders of the indicators by slender wires running over turned 

 pulleys, and all the fixtures were made as non-elastic as possible. 

 The atmospheric line was, in each instance, produced with as scrupu- 

 lous accuracy as possible. Owing to the oscillations or vibrations of 

 the pencil under such quick action, it became necessary to measure 

 the area with unusual care, and each diagram was divided into very 

 narrow spaces, to obtain the average height with great accuracy. To 

 compute the quantities of steam used, with any approximation to 

 mathematical exactness, the capacity of the clearance, the ports, &c., 

 should, of course, be added to that of the cylinder proper ; and the 

 capacity of these spaces I ascertained with tolerable care, by filling 

 the end of each cylinder with water, while the crank was near its 

 center. It should be observed, that in every instance proper allow- 

 ance was made for the steam already in this space, before the opening 

 of the steam valve, as, although neither of the engines " cushioned" 

 the exhaust steam to any considerable degree, allowance for the weak 

 steam in this space produces a sensible effect on the results. To 

 ascertain the boiler pressure, and also the pressure in the pipe near 

 the engine, new and accurate steam gauges were attached at both 

 points, and the pressure generally, while working, was only about 

 one pound less in the steam chest of the engine than in the boiler. 

 The tables given herewith show, in a tabulated form, the results, 

 deducted from twelve diagrams taken on one, and of thirteen taken 

 from the other engine. 



One point of very considerable general interest and importance 

 appears very prominently in the tables of results from both engines, 

 and that is, that while both engines vary their point of cut-off from 

 almost the very commencement to half stroke, the degree of economy 

 compared with the quantity of steam used, increases as the ratio of 

 expansion of both engines is decreased ; in other words, the economy 

 appears greater when the steam is cut off at half-stroke, and only 

 expanded to twice its original volume, than when cut off at any earlier 

 period and expanded to jf^arger extent. This would evidently not be 

 the case with engines which receive the steam more freely, so that the 

 pressure previous to the closing of the steam valve would be very 

 nearly up to that in the steam chest ; but a series of experiments 

 which shall determine the exact value of the expansive principle ia 



[Inst.] 60 



