1648. 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



U9 



These strata, iiioviug with the piston, shoiihl naturally develope 

 themselves, in order to follow and push it; and it is in this de- 

 velopment of strata, one after the other, that the steam must em- 

 ploy a portion of its force, a portion which is certainly lost to the 

 engine. The greater the numher of strata, the more rapid is their 

 development ; and the more power that is thus absorbed, the less 

 will there remain for useful effect. 



Suppose the steam introduced into a cylinder to be intercepted 

 at the moment « hen the piston has reached a fourtli or a third 

 part of its stroke, to give place to the expansion : from this mo- 

 ment we may imagine the fluid mass di\ided iuto a determinate 

 number of successive parallel strata, and lieginning to develope 

 and expand themselves to drive the piston and follow it. It will 

 then be apparent that the stratum nearest the piston will, without 

 doubt, be able to exert upon it all its effort, and all the rapidity of 

 which it is capable ; but it will be also apparent that the one which 

 follows cannot do as much, because the preceding stratum con- 

 strains it, by pushing it backwards at the same time that it forces 

 the piston forwards. By its condition as an elastic fluid, steam 

 should naturally expand every way, and maintain at the same time, 

 as is admitted, a uniform density throughout its volume — conse- 

 <juently, the stratum which dri\-es the piston on one side, repels, 

 on the otlier, at the same time, the stratum wliicli follows it, 

 although allowing itself to be penetrated liy it ; the latter repels 

 the one whicli follows it, and so on to the last, which is at the bot- 

 tom of tlie cylinder. 



There must, therefore, be a collision between one stratum and 

 anothei-, on account of the difference in their ^■elocities, and of the 

 necessary compenetration of one stratum into another, in order 

 that the'uniformity of density may be maintained. This collision 

 must evidently produce a loss of power — a loss Mhich should be 

 proportional to the differences of the velocities, and which will be 

 the more considerable according as the number of successive strata 

 is increased, and as the expansion takes place more (juickl)'. 



It is from these considerations, conffrmed by the results of ex- 

 periments upon the effect of helical springs, that I am persuaded 

 that a given quantity of steam, working by expansion, will produce 

 more disposable and useful effect acting upon a ])iston of a large 

 surface and short stroke, than upon a piston of smaller surface and 

 with a stroke proportionably longer, all other circumstances being 

 equal. Desirous of determining the truth of this opinion by rigo- 

 rous experiment, I caused two steam-engines to be constructed 

 under conditions strictly equal, and calculated to produce the same 

 dynamic effect, according to admitted principles. But in one of 

 them, the relation of the surface of the piston to the length of the 

 stroke was in an inverse ratio to that of the other : that is, if one 

 of the pistons had a surface of 20 and a stroke of 24, the other had 

 a surface of 80 and a stroke of 6 ; so that the volume produced by 

 the movement of one piston is precisely equal to that of the other. 

 There is, therefore, exactly the same quantity of steam entering 

 and leaving the two cylinders at each stroke of the piston, and, 

 consequently, when the number of strokes is the same, in a given 

 time, in each of the two engines, it ought to be certain that there 

 is the same volume of steam, in the same physical and mechanical 

 conditions, used by each cylinder. These experiments, of which a 

 table is given, were made w ith all possible precaution, in order that 

 all the conditions of the apparatus should be identically the same ; 

 they were repeated several times, on different days, and in the 

 presence of several competent persons. 



The following table shows the mean of the results obtained in 

 several series of experiments, the apparatus being always kept 

 under the same conditions : — 



The numbers in this table will show, at a glance, the difference 

 of effect between the two engines. Although every precaution was 

 taken to avoid error in tlie measurements and observations, and 

 though the numbers in the table only show tlie mean result of 

 several series of experiments, I do not assert tltat the ratios there 

 given are strictly those which should result from the physical law 

 of this phenomenon. New experiments, with engines of greater 

 power, and an exact calculation with regard to the results obtained, 

 can alone establish, with the accuracy desirable, all these relations. 

 I believe, however, that the reasoning upon which my opinion is 

 founded, and the results of the experiments which go to confirm it, 

 authorise me to make the following conclusions : — 



1. That the velocity of the piston has a much moie remarkable 

 influence upon the useful effect of steam than has been heretofore 

 supposed. 



2. That this influence is very greatly increased, and according 

 to a certain ratio, on account of the amount of expansion which is 

 allowed to the steam ; the greater being the expansion, the greater 

 is the tlifference of effect. 



3. That in order to obtain from steam the greatest amount of 

 useful effect, it is necessary to use cylinders as wide and short as 

 may be practically convenient, and that the piston should move at 

 a very low velocity. 



It is certainly not unknown that the effect of steam has a rela- 

 tion to the velocity of the piston ; but it has not yet, so far as I 

 know, been recognised that the velocity of the piston has a parti- 

 cular and considerable influence upon the effect of the expansion ; 

 and I believe myself to be the first who has directed attention to 

 this subject, and who has sought to demonstrate the truth by ex- 

 periment. Tlie numbers in the table show, iu effect, very con- 

 siderable differences, although the velocities of the two pistons 

 appear only in the ratio of 1 to i. The experiments mentioned 

 have manifested two other phenomena which have attracted my 

 attention, and wltich I recommend to the notice of scientific per- 

 sons : the first is, that in the engines which I used, and when they 

 were worked by expansion, the pistons were compelled, in some of 

 the experiments, to complete their stroke while having against 

 them (on account of atmospheric pressure) a resistance stronger 

 than the force by which they were impelled. The other phenome- 

 non is relative to the work done by the two engines. In most of 

 the experiments made, the useful effect, as measured by the break, 

 %vas always, and even considerably, more than the theoretical effect 

 of the motive power. Does tliis difference of effect tlepend upon 

 the partial vacuum created in the escape-pipe on account of the 

 rapid passage of the steam, so that the pressure upon the piston 

 has a relatively greater force ? Jlay this rarefaction in the escape- 

 pipe also account for the continuance in the stroke of the pistons, 

 although they may have been placed in equilibrium by atmospheric 

 pressure before making a half, or two-thirds, of their stroke ? 



All these questions, the impcn-tance of which will be readily per- 

 ceived by men of science, have need of study and elucidation, hv 

 experiments, perhaps of a different kind. On my own part, I shall 

 do all that I can ; but I call for tlie aid of learned persons who are 

 conversant with such matters. 



ROYAL POLYTECHNIC INSTITUTION. 



This iDstitution was re-opened at Easter, although the additioual 

 buildings are not quite completed. Whea this is done, there will be greatly- 

 increased accominodalioa, for the whole building will be doubled in size. 

 The grand theatre will be one of the largest of the class in London, and 

 capable of holding lifteen hundred persons. The screen is likewise of 

 colossal size, so that a vast picture can now be shown on it, with all the 

 resources of powerful instruments. Thus the Institution is in possessioa 

 of a dioiainic exhibition of much greater power, and possessing much 

 more variety than any in London. In the illustration of microscopic 

 objects, the great screen is likewise a valuable accessary, and its results 

 present a striking contrast to what used to be a state ol microscopic illus- 

 tration, but a few years back. The old great theatre has been very much 

 improved, and is now reserved for chemical lectures, the small chemical 

 theatre at the other end of the Institution being devoted to purposes of 

 exhibition. The grand theatre is accessible from three floors of the central 

 hall, so that it can soon be filled and cleared, which is a great convenience 

 to visitors. Over the screen room is a kind of bazaar of objects of orna- 

 mental art, consisting of porcelain and glass works, of great merit, from 

 the establishments of Alderman Copeland and others. They show, evea 

 more effectively than the exhibition at the Society of Arts, the great pro- 

 gress of these important arts in England. When the front of the Royal 

 Polytechnic Institution in Regent-street is finished, it will make one of the 

 finest buildings in Ihe street, and a very great ornament to it. The altera- 



