1839.] 



THE CIVIL ENGINEER AND ARCHITECT\S JOURNAL. 



457 



ON THE ELASTIC FORCE OF STEAM. 



Obscrralioiiif of M. Arago on a Memoir by Mr. Fakey, uii Ik ilaiitic 

 force o/Sl('am,p/ibli!iked in the /rsl rohtme of /he " Tran^aclions of Ik 

 iMtitiilkm of Civil Engineers." 



(From the correspondence oftk Acadeiuie des Sieiices, at Paris.) 



In presenting to the Aciideiiiy the first volume of tlie Tninsar/ions 

 of a society wliose hil)ours miiy be expected to exercise ;i beneficial 

 iiiHnence on the progress of the art of the engineer, M. Arago ex- 

 jjresses his regret at having foiind therein a memoir by Mr. Farey, 

 which certainly does not deserve a place in so useful a collection. 

 This memoir is a bitter and unfounded criticism on a work of M.M 

 Dniong and Arago, midertaken by order of the Academy, and honoured 

 with its approbation. M. Duiong had knowledge of Mr. Farcy's 

 memoir a short time befiu'e his death ; he was very m\ich grieved, and 

 proposed to refute it. What my illustrious frienil was luiable to do, 

 said M. Arago, I shall now attempt. The task will, however, not be 

 ditlicult; it will suffice to compare the following quotations from the 

 two works : — 



"The principal object of the present commimication," says Mr. 

 Farev, "is to show the coincidence between Mr. Southern's scale (for 

 llie elastic force of skum corresponding to different dc grits nf Ik tkr- 

 »iOHit/f/-,j anil that of a new series of experiments made in Paris in 

 JS29, by a Committee of the Academy of Sciences." 



"Another object of the communication is to put on record, in the 

 papers of tlie Institution, a memorial of the fair claim of our country- 

 man, Mr. Southern, to the merit of priority in accurate determination 

 of this law, in opposition to tlie unfoiindtd asserlion of the Frencli 

 author (M. Duiong) who has just published the new experiments, 

 according to which //(e prtrious dtttrminations in England inreirru- 

 VfOiis. Mr. Southern's determination is not mentioned in this sweeping 

 condenniation. * * * 



" f. * At four atmospheres Mr. .Southern found 



the temperature 29;3-ll degrees, and the acadendcians 'IW.iu. This 

 last is not an accidental coincidence, but an adoption of Mr. Southern's 

 scale, through Mr. Tredgold, t/iuugh not acknon^lcdgid as xncli." 



"in adopting this fornuda from Mr. Tredgold, (who quotes Jlr. 

 Southern's experiments, and takes them as his basis,) the French aca- 

 demicians could not have been ignorant of Mr. Southern's deternd- 

 nations, nor of their accuracy * » * 



" Under tkse circumstances, it was not candid that all mtntion of 

 Mr. Soulhern's deterniinatiuns should hare been suppressed. * 



" * It is to be remarked that the elasticities 



were measured bv the French academicians by the conqjression of 

 air included in a manometer, and not by a direct measure of a culunm 

 of mercury, or a loaded safety valve ; whereas Mr. Southern used both 

 those means, and employed very correct thermometers, and therefore 

 his scale is of as nuich authenticity as that of the French. * * 



" * * As there is no certainty in thi! exactitude 



of either temperatures or elasticities when so great as i'iH degrees 

 and 24 atmospheres, it is not advisable to adopt a new law of pro- 

 gression for the sake of reconciling differences of 2A degrees from 

 uncertain observations." 



The imputations of Mr. Farey, said M. Arago after having made 

 the above quotations, are categorical and numerous. In order to 

 refute them, I shall content myself with quoting several passages from 

 the memoir criticized. 



The author makes M. Duiong assert that the English determinations, 

 previous to the experiments which he made with M. Arago, were 

 inexact. The following is the passage in the Report to the Academy 

 to which Mr. Farey alludes: — 



" La science ne possedait que des mesures assez discordantes au- 

 densous de huit atmosphi'res, et pour des pressions plus fortes, absolu- 

 ment aucun resultat d'experiences directes."* 



Now in this passage there is no mention either of English, French, 

 or German. It is merely stated that the results of the known experi- 

 ments were discordant, and that it was ditlicult to choose ; and this is 

 an incontestable truth. 



The great crime of the French Acadefmicians, in the eyes of Mr. 

 Farey, is to have suppressed all mention of Mr. Southern's determi- 

 nations. It is in this that they have been wanting in candour. 



To show how we have suppressed all mention, and how we have 

 been wanting in candour; the following is a passage of the Memoir. 

 The reader will hardly believe his eyes: — 



" Les determinations seules de Southern et de Taylor offrent avec 



'■ Science possessed but discordant measures below eight atmo.yiheres, and 

 for greater elasticities ab.solutely no result of direct experiment, 



celles-ci (les determinations Fraucaises), une conformitc d'aulant plus 

 Irappante ((u'elles out etc fonrnies par un mode d'observation totlile- 

 ment different. A I'epoipu* ou nous avons calcule la table inserce an 

 rapport provisoire cite plus liant, nous les considerions dejii comme 

 les plus vraisemblables ; aussi ne tronvera-t-on entre cette table et celle 

 que nous alloiis donner, que des differences presqu'insignificantes, dans 

 la partie de I't'chelle qui leur est counuune." — ( Mi'iu. de I', /lead, de 

 Sciences, t. X, p, 222.)>' 



This is not all. Mr. Southern has been quoted, not only for his 

 experiments, but also for a simph; fornuda of interpolation, which the 

 following extract will prove : — • 



M. Young parait ctre le premier qui ait employe le mode d'inter- 

 polation, qui consiste a reprcsenter les forces elasticpies de la vapeur, 

 par une certaine puissance de la temperature augmentee d'un nombre 

 constant. M. Young avait trouve ([ue I'exposant 7 satisfaisait aux 

 experiences connues a I'epoipie de la publication de son ouvrage. M. 

 Creighton prif I'exposant ti, ((ui lui parut mieux s'accorder avec les 

 rc'sultats du Dr. Ure. Mr. Southern adopta le nombre rrl3, qu'il de- 

 termina sans doute par tctonnement. jVIr. Tredgold retablit I'exposant 

 de Creighton, en changeant le coefficient, &c.. — -(JMi'm. de V Acad. t. x, 

 p. 230.) -i- 



It is unpleasant to have always to answer by formal denials; but is 

 it my fault '. Mr. Farey says that for four atmospheres we took Mr. 

 Southern's deterndnation, without aeknoii'ledging it. This is not the 

 fact : our observations embrace the interval between one and 24 at- 

 mospheres, it was therefore unnecessary for us to borrow anything 

 from any other authcu' ; but, as our observations did not correspond to 

 round numbers of atmos]iheres, we could not do otherwise, in con- 

 structing the table, than make the interpolation by means of the for- 

 nuda which gave the nearest approximation to our results. This 

 fornuda was Mr. Tredgold's. M. Duiong stated thns nuich, and he 

 had nothing more to aeknowlidge. 



Once in the vein of detraction, Mr. Farey has not confined himself 

 to the historical part of the Report. The experiments of the Com- 

 mission, examined in their details, appear to him defective. And is 

 it not true that MM. Duiong and Arago did indeed make use of a 

 manometer? 



But wdrat would have become of this pretended difficulty, if Mr. 

 F;irey had added that the manometer was graduated directly, by mer- 

 curial columns wdiich attained a height of 20 metres (nearly t)(j feet) ? 

 Can the English critic have mistaken the object of the Academicians? 

 The reader may judge by the following extract from page I9j of the 

 Memoir : 



"(La Commission) s'est dctermince a recourir au moyen le plus 

 pciuble, raais aussi le plus exact, la niesure directe de la colonne de 

 mercure capable de faire eqnilibre a I'clasticite de la vapeur! " % 



Mr. Farey ))refers the determinations of Mr. .Southern to those of 

 M.M. Duiong and Arago, which he is certainly entitled to do as far as 

 regards the numbers comprised between and 8 atmospheres ; but 

 beyond this he must, nolens rolinf, refer to the French measures, since 

 our critic's compatriots have not determined any elastic force above 

 eight atmospheres. M. Arago remarked that the difficulties and 

 extreme danger of the experiments commence at 10 atmospheres, and 

 that the Commission carried theirs as far as 24. 



Mr. Farey prefers the English deterudnations, because Mr. .Southern 

 enqjioyed Very exact tkrmomiters. What then ? Can it be supposed 

 that a Commission, prosecuting its researches under the auspices of 

 the Academy, a Commission which counted among its members, which 

 had for reporter one of the two authors of the excellent memoir, now 

 become classic, On the cuniniunieatiou of Heat, can it be supposed that 

 such a Commission shoukl not have used nry exact thermometers ? 

 Such doubts, when they are gratuitous, when they are not founded on 



* Tliedeteniiiiiutioiis nf Sinahcrn aiitl Tinjhir alone shew a conformity with 

 these (the Fri-nch determinations) l)v so much die more striking, ns they 

 Here furnished by a lotallv ililiVrenl'mmlc of observation. At the time when 

 we ra/c«/(7/c</ thc'lable nliuli was inserloil in the provisional report quoted 

 above, «"e alre.idy considered them as the most iirobal.le ; anil the iliflbronecs 

 between that lal/le and tlie one we are abnut to give, will be fbund to be \ery 

 incuiisiilenible in the pari of the scale «liicli is lommon to both. 



r Mr. Vuuiig appears to have been the (irst «ho made use of the method 

 of interpolation. «hich consists in n-preseiuing the elastic fiaxc uf s:eain liy 

 a certain puuer of the IpiiquTiliire augmenleil by a cimstant nnmlvr. Mr. 

 Vnung had fuund that tlu> index 7 made ibc results cuiniide ttitb the experi- 

 ments »hieh »eie knuwn at the time his work «as piiidished. Mr. Creighton 

 took the index 0, wbicli seemed to liim to iigne betler with Dr. Ure's results. 

 Mr, Suuthern :ido|ited llie number .')'i:!, wliR-lihe no dnnbt determined by 

 liltoiiHciiinit. Mr, IVedguld resumed C'reiyliton's index, and changed the 

 ciielheient, S<'C. 



; ('the Commission) resolved to have recourse to the most laborious, hut 

 at ihe same time the most exact method ; llie direct measure of the column 

 of mercury capable of supporting the pressure of the steam ! 



2 R 3 



