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on lli: DC PYO~Y’S Broke, or Fricbion f&namomctn. By WmLXAM STEWART.

I beg bve to oiler you the follonring observations on the articlerelative IO hl. de Prony’s friction dynamometer, given in your pages[or #ctober and December last.

I entirely agree with you in respect of the utility, the simplicity, theelegance, aud the accuracy of that mode of measuring the mechanicalpower of prime movers; bur I think that the. description you havegive11 of the form, and more especially of t,he dimensions of the brake,en~pioycd by Captain Morin, in his experiments on water-wheels, i sill calculated by reason of the expense incurred, to briug into generaluse au instrument which seems to me about as necessary in the saleand purcltase of prime movers, more especially steam engines, as thecommon bahace is in the sale and purchase of grocery goods.

Captain Moriu, without doubt, made (he brake which he empioyed,purposc’iy of’suffcient strength, to be able to accomplish without ac-cidellt, or repairs, (he series of experiments which ho intended tomake; and, to make sure of this, 110 expense was spared, no secondconsideration, such as the trouble and expense of carriage, occupiedhis mind,-and! as you inl’orm US, ‘&the whole did not exceed 500lbs.” Fronl ths statement, we may presume, that the weight of ironit1 the composition of the brake was uearly, perhaps altogether, 500Ibs.; the esptwsc of which, iucludillg the workmanship, may havebeen Y 16; but very few persons XVIII be disposed to lay out such asum to try an experiment, which, to many, may not appear decisive,and 10 others it may appear uuneccssary. It is certain, that in ordi-uary cases, sill an expcuse is utincccssary, as will appear hereafter,

In 132:, I was, from accidental circumstances, charged to clean,examitic, aud repair, if necessary, the steam engine on which M. deProny peri’ormed the first esperimcnt with his friction dynamometer;I may state, in confirmation of what I here advance, that in execut-ing my t:lsk, I put two new cast-iron tubes of 9 or 10 inchesdiameterinto the boiler, the old tubes having been left full of water, and beinguncrnployed during the severe winter of January, 1821, were rent bythe congelation of the water, nearly from one end to the other, in adirection parallel to the axis. Most of the joints in the boiler hada190 to lw reuewed; all the working parts of the engine were tabapart, cleaned, carefully examined, aud with equal care returned a&piece IO its place. Nothiug was wanting in the engine except a fSWspriugs to be changed in the metallic piston of the steam cylinders;llrc repairing of a few joints, and, what was more essential, the fillingup of a blown hole, nearly au eighth part of an inch in diameter, inthe steam cock of one of th cyliuders, which was so situated CM toadmit tllc steam from the steam pipe, comnlunicatin.g with the boilerIII~O tl~e steam cylinder 011 the opposite side of the pdon, at the timeit ought to 11ave been exhausted.

Apart from these small exceptions, the engine, in all its parls, wasof the very best workmauship, csccuted at Messrs. Ferrier and Ed-

X56 Civil En@neering.

wards’ rnanufactory at Chailot, in Paris. The engine was situated inGros Cailau, in Paris, on the left bank of the Seine, and destined toraise water for the use of the city.

The engine was of the high pressure kind, with two cylinders, onthe principle invented by Mr. Woolf, and was constructed under thedirection of Mr. Edwards? previously a partner with Mr. Woolf, underthe firm of Woolf & Edwards. The engine was sold to Mr. Ferrier,director, and, I believe, proprietor of the said water works, as equiv-alent to 20 horse; power, but was found by M. de PVO~~Y to be equalto 16 horses power only.

From this experiment I infer, that there is error and exaggerationin the supposed advantage of double cylinders, with expandmg steamon Woolf’s principle. ‘I’he firm of Ferrier & Edwards never wouldhave sold an engine for a power greater than that which they believedit to be capable of giving-and previous to this experiment of M. deProny, there was no method other than vague calculation to ascertainthe power of steam engines.

These details which have not hitherto been published, and whichperhaps none but myself could give circumstantially, may be interest-ing to many of your readers, inasmuch as they are connected withthe subject of your article before alluded to, and with the first appli-cation of the friction dynarnomcter, Other reasons, however, havedetermined me to communicate them, viz., lst, because it is univer-sally agreed, that in performing any experiment, every circumstanceconnected with the experiment. ought to be noted, both before andafter, as well as during the experiment; for the reason that sometimescircumstances apparently unimportant at the time, may afterwardslead to important results. znd, because I consider the experiment byM. de Prony, on the steam engine, at Gros Cailau, as being decisivein showing the fallacy of the estimated power of double cylindersteam engines, as introduced by Messrs. Woolf 82 Edwards. It is ofimportance to state, that nothing was wanting to render the experi-ment complete; the engine was perfectly new, having been at workonly a few days after Its erection-that it was most efficiently con-structed, and highly finished- t h a t immediately preceding the experi-ment it was carefully examined, cleaned, andrepaired, where wanted,in all its details, and that for the express purpose of being tested byM. de Prony, and by order of’ a legal decision of one of the courts oflaw in Paris. 3rd, because this experiment is a striking example,showing how it happens, that from small, and apparently, triflicg,causes, sometimes great results are obtained ; in this case, 1 am satis-fied in my own mind, that had the small blolun hole already statedto have been discovered in the steam cock not existed, (which holemust have reduced the power of the engine, at least, one-half,) it isprobable that no discussion between the buyer and the seller of theengine would have taken place,-and that M. de Prony would nothave been called upon to test the power of the engine; and, con@-quently, on that occasion, at least, his attention would not have beendirected to the investigation of the measurement of the force of primemovers, and his ingenious and simple dynamometer might have been

Proceedings of the lizstilution of CiviZ Engineers. pi61

a~ yet unknown. And 4thly, allow me to add that I feel prolid ~1beiilg able to associate my name with a circumstance which gave ok_casion for an invention, characterized by you “as one of the Jmwtelegaut and useful inventions which France has bequeathed to pmc-tical mechanics.”

But, to resume, the brake, or friction, bahd employed on this wea_siou by M. de Prony, did not, like that of Captain Morin, weigh 500lhs. Its weight did not exceed 25 Ibs., neither was it necessary thatit should.

Since that time I have had occasion, in contested cases, to employthe friction dynamometer, and can assert that, when applied to testthe power of steam engines of twenty horses’ power, the iron workmay be made under 20 Ibs. weight, and the whole expense not exceed~1. For water-wheels with wooden shafts, a larger and strongerfriction-pulley and band is necessary; but even for that, in ordinarycases, 100 lbs. weight may be suff’cient. An expensive link-chain isnot essentia!ly necesssary : a bar of flat iron, or strong sheet-iron,bent, will answer equally well, and the cost not exceed &!4.

Four sets of setting screws, for the ring, or pulley, is preferable tothree sets, as every practical mechanic knows. L4 plate of iron oughtto be put between the point of the set screws and the wooden shaft,and the open space between filled with wooden wedges, as describedby Captain Morin.will suffice.

For cast-iron shafts, a Very small friction ringThe expense will be reduced in proportion,

From what has been here stated, it appears certain that the practi-cal operation of ascertaining the power developed by prime movers,by the USE of M. de Prony’s dynamometer, is as simple, as easy, andas little expensive in its application as you have shown the calcula-tion to be.

LiverpoaZ, March 12, 18-14. Ibid.

Proceedings oJtheInstitution. of CivilEngineers.--From theLondondthenaum.

dtmospheric Railway-Juue 4.- T h e applicability of the systemof the propulsion of railway carriages, by the pressure of the atmos-phere upon a piston, traveling within an exhausted line of pipes, hasoccupied a portion of three meetings ; and, although much time hiisbeen devoted to the discussion, it cannot be said that any positiveconclusion has been arrived at, Indeed, when it is considered thatthe system has only been tried upon a line peculiar in its local&y, inits steep gradients, in the engine having only to exert power indraw-ing the carriages in one direction, and their descending by their owngravity, and in the trains being only required to run a distance of amile and three quarters, without stopping at any intermediate station,it may be argued, that although, as is evident from the testimony ofthe several speakers, great results have been obtained, it is scarcelypossible to infer what the results would be on lines with gradients in

vow. VIII, 3a.11 SEBXEE, No. a,--SEPTEXBZB, 1844. 14