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THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[Al'GUST' 



suspentlcd nt the other end; further, let /; A- represent an inflexible 

 circular phite fitting air-light into the cylinder, liut supposed to huve 

 no weight, and let there be a space c between this plate and the piston 

 ;; filled with air of a given density. 



The piston p being near the top of the cylinder, the circumstances 

 are analogous to these of the Cornish single-acting engine jnst before 

 the conunencement of the working stroke, the air in the space c above 

 the piston representing the cusbion of steam against which the piston 

 is brought to rest at the end of the return stroke, the beam h the 

 balanced portion of the moving parts of tlie machinery, and the mass 

 M the unbalanced portion. 



In order to form an idea of the manner in which the momentum of 

 the steam, admitted suddenly above the piston at the beginning of the 

 stroke, is transferred to the latter, and thus increases I be effect above 

 wliat is due to the simple elastic force of the sleam, let a mass S (say 

 equal to the mass of steam admitted in the stroke) im|)inge against 

 the plate h k with a given velocity. The result of this impact is, ob- 

 viously, that the mass S loses a portion of its velocity, and consc' 

 quentiy of its momentum, which is transferred to the air contained in 

 the space c, which in its turn, comnuniicates the chief part of this 

 momentum to the piston y;, the beam b and the suspended mass M. If 

 the mass S be supposed to strike the ])late h k with a velocity equal to 

 that of the steam at its entrance into the cylinder of the Cornish en- 

 gine, its percussive effect may be assumed to be the same as that of 

 the latter, though it will in reality be greater on account of the mmul- 

 tantoua action of the whole mass, whereas the mi'ss of steam airives 

 in the cylinder gradually. The inter|)osition of the air e is essential 

 to the perfect conformity of the two cases, for the entering steam no 

 sooner passes the contracted orifice of the throttle-valve, where it im- 

 pinges, as it were, against the steam already in the cylinder, than it 

 expands and loses the greater part of its velocity, at the same time 

 compressing the steam with which it mingles. 



The necessity of adopting this theory (of the Percussive Action of 

 Steam) was forced upon i\lr. Farkes liy his inability to discover, in the 

 simple elastic force of the steam employed, an amount of power ade- 

 quate to accomplish the actual duty ascertained to have been performed 

 bv several of ihe Pumjiing Engines in Cornwall, the facts observed 

 admitting of no question. This is, in our ofiinion, the only valid 

 argument brouglit forward by the autlior in its favour, though lie has 

 adduced several others in corroboration, which, however, re(]uire the 

 co-existence of the former to give them weight, and even so they are 

 but of a negative character, amounting in suljstance to this: since the 

 amount of power due to the steam's elasticity alone is less than tlie 

 amount of work done, an additional (piantitv of power must be derived 

 from other source ; and whence can it be derived but from the instan- 

 taneous action transmitted to the [liston, on effecting the sudden com- 

 munication between Ihe steam in the cylinder and tiiat in the boiler. 



Assuming the data furnished by exi)erinient to be correct, (and we 

 have no reason a prion to doubt their accuracy), the above reasoning 

 appears to be conclusive, at least in so far as the additional power re- 

 quired to realize the observed dynamic effect must be sought in some 

 property of steam distinct from its elastic force ; and its Alomentum, 

 or rather Inertia, is the only property which suggests itself as capable 

 of supplying any additional amount of power. 



Admitting, therefore, the inadequacy of the simple elastic force of 

 the steam to accomplish the work actually performed, and assuming 

 the ileliciency of power to be supplied by the Steam's Percussive 

 Action, the next step naturally, is to examine into the causes and 

 effects of this action. 



The cause is obvious, and is almost sufficiently explained in the 

 illustration which we have given above. A mass of dense steam passes 

 through the throttle valve at a great velocity, the chief part of which 

 it loses by the time it comes to act by its ehistic force upon the surface 

 of the piston. This mass of steam must, therefore, in losing its velo- 

 citv, impart its momentum to some other body or bodies, through the 

 medium of which it may afterwards be utilized in increasing the dyna- 

 mic effect of the steam. The body which receives the shock of the 

 entering steam, and transmits its momentum to the moving parts of the 

 engine, is the steam cushion represented in the illustration by the air 

 c, and tlie entering steam by the mass S. It is this imiiarting of its 

 momentum which is called the Percussive Action of the steam. 



In investigating the elects of this action, our object is to ascertain 

 the amount of momentum transmitted thereby to the piston and other 

 pieces of machinerv connected with it, and we looked in vain to Mr. 

 Parkes' work for assistance in this inquiry ; there is, indeed, an article 

 (page 209), headed: Dclcrmi nation of the quantity of Ptrcuimite Ac- 

 tion, which commences with the statement that " the dynamic effect, 

 or quantity of action, due to percussion, is discoverable, and assignable 

 for each example : but the only method employed by the author for 

 its determination is that of elimination, that is, by deducting from the 

 total dynamic effect of the steam found in the quantity of work done, 

 the amount due to its elastic force, including expansion, the remainder, 

 which is the deficiency of power accoriling to the ordinary theory, 

 being attributed to the percussive action. In a note at the foot of the 

 same page Mr. Parkes says : "It forms no part of my task to investi- 

 gate tlie abstract question of the quantity of this species of force to oe 

 obtained from steam ; my present purpose is confined to the determi- 

 nation of the eff(?ct attributable to it in the three engines subjected to 

 analysis." It is a pity he did not make it part of his task to investi- 

 gate, not tht abstract question of the qiiaiititi/ if this spcctts of force to be 

 ohtatutdfrom steam, but the practical question of thecpiantity which the 

 steam would afford in the three cases under consideration. This in- 

 quiry would, doubtless, have been full of difficulties, should the result 

 not turn out to be equal to o, wdiich »ve much suspect would be the 

 case if the investigation were based on the laws of percussion as laid 

 down in tlie treatises on Mechanics ; and if the new principle is to be 

 established in opposition to these laws, it is necessary first to demon- 

 strate their fallacv; but that this is not the view of the case taken by 

 the author is evident by his merely comparing the ])ercussive action 

 of the steam to the shock of a solid body, without intimating in any 

 wav that the laws laid down for solid bodies do not obtain equally 

 with regard to steam. He overlooks, however, the important con- 

 sideration that the shock of the entering steam is not received imme- 

 diatelv by tlie piston, but by the steam previously occupying the space 

 above it, and likewise that the reaction is necessarily equal to the 

 force of impact. 



By reason of this latter condition the entering steam can only part 

 with its momentum as fast as the steam in the cyliniler is capable, by 

 its Simj,li tliistic force, to oppose a resistance, or reaction, equal to the 

 force of percussion. The latter is therefore always strictly measured 

 bv the elasticity of the steam in the cylinder, the dynamic effect of 

 which thus includes that due to the Percussive Action. If, then, the 

 indicator diagram exhibits a faithful representation of the elastic force 

 of the steam as it varios from the commencement to the termination 

 of the stroke of the ]>iston, it must necessarily furnish us with the 

 means of determining the whole amount of dynamic etl'ect which can 

 be obtained therclVom. It may also be observed that the effect of the 

 Percussion is transmitted, " by the properly of fluids," to the Indicator 

 piston as well as to the woiking piston, so that, even if there were a 

 Percussive force which acted on the latter in addition to the elastic 

 force of the steam, its iiiHuence, being felt by the former also, would 

 be indicated in the diagram by an additional elevation of the 

 pencil. 



The above discussion convinced us, as it may also some of our readers, 

 and [lerhaps Mr. Parkes himself among others, th.it the difference ob- 

 served between the amount of power due to the elactic force of the 

 steam and the duty actually performed by the engines subjected to 

 analysis cannot be attributed to the Percussive Action of the steam; 

 and, as there appears to be no other source of power to which it can 

 be ascribed, we are compelled to conclude that the supposed difference 

 does not exist in fact, and consequently that either tlie experimental 

 data, or the calculations based upon them, are erroneous. 



We have said above that we had no reason i) priori to doubt the 

 accuracy of the observations, and we will therefore now examine into 

 the details of Mr. Parkes' calculations relating to the Uuel Towan en- 

 gine, with the view of discovering whether the discrepancy ob- 

 served between the power <leveloped by the simple elastic force of 

 the steam and the actual work done be not attributable, either wholly 



