304 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Skptembhk, 



the momentum of the different parts of the engine, the pump roil, 

 and the column of water set in motion ; we must come to the conclu- 

 sion that iis the pressure on the piston varies without any medium to 

 equalize its effect, the resistance opposed to the pump plunger ought 

 to vary also. 



In the common single acting pumping engine this medium is in a 

 measure supplied hy the weight of the pump rod, which is made suHi- 

 ciently heavy to overcome the friction of the engine, and to raise 

 the piston at the relurij stroke, and by the momentum of the beam 

 and other parts of the engine ; and also by the momentum of the 

 column of water before it enters into the air vessel ; these however 

 would form but a very small reservoir for the immense pressure at the 

 conjmencement of the stroke, so that in these engines the application 

 of the percussive force must he very limited, in consequence of the 

 pump rod not being of a sufKcient weight to accumulate all the over- 

 plus power at the commencement of the stroke, so as to impart it to 

 the plunger when the pressure on the piston, in consequence of the ex- 

 pansion of the steam, falls below the resistance on the plunger so that 

 the percussive force would be in a great measure entirely wasted. 

 Again on the other hand, if the rod which generally weighs from 8 to 

 9 tons, were made heavier, so as to equalize to a great extent the pres- 

 sure on the plunger, it would be more than necessary to overcome tlie 

 resistance of the engine at the return stroke, and so occasion a loss of 

 power. 



The action of the Cornish single acting engine is somewhat different 

 from that of the common one, the pressure of the steam on the piston 

 instead of being applied directly to work the pumps, is applied to 

 raise a pump rod of sufficient weight to work the pumps at the return 

 stroke. The result of this difference of arrangement is that instead 

 of having a pump rod of S or 9 tons weight, we get one of from 20 to 

 70 tons weight according to circumstances. Here then we get an im- 

 mense mass of matter amply sufficient to accumulate all the overplus 

 pow er at the commencement of the stroke, and to return it as required 

 at the end of it. The action of it is this : the steam being admitted 

 suddenly into the cylinder strikes upon the piston at rest with a con- 

 siderable force above what is due to its elastic pressure alone, and sets 

 this massive pump rod in motion ; the steam in the cylinder expands, 

 and consequently acts with less force on the piston, and the pump rod 

 after the pressure of the steam on the piston, becomes insufficient of 

 itself to raise it any higher, assists to carry itself through the remainder 

 of the stroke, by means of the power that it accumulated at the com- 

 mencement. When the rod is thus as it were thrown up to the top 

 of its stroke, the equilibrium valve is opened and the weight of the 

 pump rod descending acts upon the plunger of the pump and raises 

 the water. In this engine then we have the means of applying the 

 percussive furce of steam to almost any extent in consequence of the 

 weight of the pump rod, which acts as a reservoir for the power that 

 would otherwise be wasted. 



These remarks I think have clearly shown that in the common double 

 acting engine the percussive force of steam could not be made to act 

 with any advantage, but would, on the contrary, occasion an additional 

 wear and tear; that in the locomotive it would act still worse, and 

 w ould actually impede the engine, if not stop it altogether ; that in the 

 common single acting pumping engine it could only be brought into 

 viseful action in a small degree ; and that in the Cornish engine we 

 might use it as a moving force to a very considerable extent. 



When we consider the amazing quantity of work done by the Cor- 

 nish engines as compared to any other, we are perfectly at a loss to 

 account for the difference, and are brought to the conclusion that there 

 must be some force in the steam which can only be applied to any 

 considerable extent in those engines, and which will not allow of being 

 so applied in any others. The elastic force of steam can be appliea 

 in any sort of engine, the expansive force can be applied economically 

 in all, but more so in the Cornish than any other engines ; but still this 

 is insufficient to account for the difference of the amount of duty done. 

 The only other force that we can conceive the steam to possess is that 

 which Mr. l^arkes has denominated its percussive force. If the con- 

 clusion drawn from the preceding remarks be just, we see that the 

 Cornish engines are the only ones in which this force could be applied 

 to any considerable extent. 



It also stands to reason that if this force does exist in the steam, 

 and if it was usefully applied it would increase very considerably the 

 duty done. It is also now a fact well ascertained that the Cornish en- 

 gines will do three times the duty of any other with the same expendi- 

 ture of fuel. 



is it not then reasonable to infer that as the Cornish engines are the 

 only ones in which the percussive action could be cm|)loycd to any 

 considerable extent, and that they alone jierform that additional work 

 that would be the effect of this force if usefully ajjpiied, we may safely 

 conclude (if all other evidence was wanting), that this percussive force 



does actually exist in the steam, and that it in a great measure will 

 account for the quantity of work done by these engines. 



Hoping that these observations may help to throw a little light on 

 the suliject, and may induce some of your readers who may have the 

 means, to pay a little attention to the'subject. 



I remain. Sir, your's, respectfullv, 



C. s. 



Danksiile. Sonlhrnnrk, 

 August 13, 1641. 



ON THE MOMENTUM PROPOSED BY JfR. JOSIAH PARKES, 



AS A MEASURE OF THE MECHANICAL EFFECT OF 



LOCOMOTIVE ENGINES. 



By the Count De Pambocr. 



In- the Transactions of the Institution of Cicil Engineers, vol. Ill, Mr. 

 J. Parkes has published a paper On Slt,am-buikrs and Sltam-tngines , 

 in which the object s to propose, as a uew measure of the mechanical 

 effect of locomotive engines, what he calls the momtntum produced by 

 the engine, that is to say the product of the mass, in tons, of the en- 

 gine, tender and train, multiplied into its velocity, in feet per second. 

 According to him, this momentum being measured at one velocity, for 

 a given engine, the effect of the same engine, at any other velocity, 

 will be immediately deduced from it by a single proportion (page 130), 

 without troubling one's head about the inclination of the road, the 

 friction of the wagons or the engine, the counter-pressure due to the 

 blast-pipe, the resistance of the air, or, in fact, any of the resistances 

 realU' encountered by the engines. 



To establish this new idea, Mr. Parkes' first step is to represent as 

 altogether faulty and impossible every calculation or experiment made 

 by others, to take account of the divers resistances offered to the mo- 

 tion of the engines. With this view he enters into a long and male- 

 volent discussion on the experiments of our Treatise on Locomotive 

 Engines, and on all the experiments on the same subject published by 

 different engineers; and to demonstrate the difficulties insurmountable, 

 in his opinion (page 124, 129), and the uncertainty attending such re- 

 searches, he indicates several verifications which, as he says, these ex- 

 periments ought to satisfy, and which they do not satisfy. As Mr. 

 Parkes gives on the subject a great number of arithmetical calculations, 

 the errors of which are protected against detection by the heap of 

 figures presented, we shall first enter, with some detail, into the ex- 

 amination of his pretended verifications, and afterwards shall discuss 

 the value of the new measure proposed by him to represent the me- 

 chanical effect of locomotive engines. 



On seeing the fundamental errors on which his reasoning and his 

 calculations are grounded, the inaccuracy of his criticisms and of the 

 results at which he has arrived, will be at once recognised. 



1st. Mr. Parkes proposes to calculate the pressure at which the 

 steam was necessarily expended on the cylinder of each engine sub- 

 mitted to experiment, m order afterwards to compare that pressure 

 with the pressure resulting from the sum of the different determina- 

 tions of resistances exerted against the piston, according to the treatise 

 on locomotive engines. With this view, he seeks, from the velocity of 

 the engine, the number of cylinders full of steam which were expended 

 per minute. Comparing tlje volume thus obtained to the volume of 

 water vaporized in the boiler, he concludes the relative volume of the 

 steam during its passage into the cylinder; and finally, recurring to 

 the table of the relative volumes of steam under divers pressures, con- 

 tained in our Theory of the Steam Engine, he concludes the pressure 

 which the steam must necessarily have had (page 82, &c.) This is 

 conformable to our theory developed in the Treatise on Locomotive 

 Engines, which, in fact, Mr. Parkes entirely adopts. But to perform 

 this calculation, Mr. Parkes takes the a rerage velocity of the whole 

 trip from Liverpool to Manchester (page 85, and table viii., col. 10; 

 table xiii., col. 9 ; table xvi., col. 2, &c.), and from that velocity he 

 pretends to deduce the mean pressure of the steam in the cylinder 

 during the same trip. Now it will be easy to prove by an example 

 that this mode is altogether faulty. 



Suppose, in effect, the engine Atlas has travelled a distance of 30 

 miles in an hour and a half, vaporizing 60 cubic feet of water per hour. 

 As the wheel of the engine is 5 feet in diameter, or 15-71 in circum- 

 ference, as there are two double cylinders-full ot steam expended at 

 every turn of the wheel, and as the capacity of those two double cylin- 

 ders, including the filling up of the steam ways, amounts to 4-393 

 cubic feet, it follovrs that the volume of the steam which passes into 

 the cylinders per mile performed, or per distance of 5260 feet, is 



5280 

 15-71 



X 4-398= 1478 cubic feet. 



