1840.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



|:}'5 



PROCSEDINQS OF SCIENTIFIC SOCIETIES. 



INSTITUTION OF CIVIL ENGINEERS. 



SESSION, 1840. 



Jan. 



14. Joshua Field, V.P., in the Chair. 



" Observations on the efficiency or gross power of Steam exerted on the 

 piston in relation to the reported diiti/ of Steam Engines in Cornwall, at dif- 

 ferent periods. By John Scott Enys, .\ssoc. Inst. C. E. 



The ailvantages wliicli may result from the union of scientific and practical 

 knowledge in the appUcation of steam power, particularly with reference to 

 the limits of gross power, are great, as likely to clieck the extravagant no- 

 tions entertained by some with respect to the farther increase of duty, and 

 to remove the disbelief of others with respect to the amount of duty actually 

 performed. 



The hmit of duty for atmospheric steam may be readily arrived at, as was 

 done by Mr. Davies Gilbert in 1827, by estimating the weight of water which 

 would rise 34 feet into a vacuum formed by the condensation of steam of at- 

 mosi)heric strength ; whence it appears, that a higher duty than 30 millions 

 cannot be obtained by atmospheric steam, 14 cubic feet of water being eva- 

 porated by a bushel of coal. Tredgold, in the first edition of his Treatise on 

 the Steam Engine, published in 1827, adopted the simpler method of multi- 

 plying the volume of steam of atmospheric strength by the pressure, for the 

 measure of the efficiency. This principle may be extended to measure the 

 efficiency of steam at liigher pressures, as the author has shown in the first 

 annual report of t)ic Cornwall Polytechnic Society ; and an extended table to 

 ten atmospheres is appended to this commuuication. 



The author then proceeds to show, that the Cornish engines are worked 

 under conditions such that a large proportion of the expansive action of the 

 steam is available on the piston, and calls attention to two necessary correc- 

 tions — 1st, for the deficiency of water in higli steam cut oft' at ntth of the 

 stroke ; and 2d, for the increase of temperature of the steam during expan- 

 sion in the cylinder, as derived from the steam jacket. The experiments of 

 Mr. Wicksteed, confirmatory as they are of the very extended experiments 

 made by Woolf at Wheal Alford, show the importance of this latter correc- 

 tion. Some error has also arisen from the use of the boiler pressure. The 

 exact determination of the pressure in the cylinder is difficult, and the oidy 

 recorded eiperiments are those by Mr. Henwood with the common indicator, 

 and (luljlished in the second volume of the Transactions of the Institution. 

 The indicator is liable to show a pressure higher than that actually exerted 

 during the expansion, Init it may be relied on for comparative results ; and 

 very accurate experiments made at the Consols by a mercury gauge, the en- 

 gine being stopped at dift'erents parts of the stroke, are said to confirm the 

 reliance which may he placed on the indicator. 



The quantity of water evaporated was very imperfectly recorded ; it was 

 stated by Watt as from 8 to 12 cubic feet per bushel, and at present may be 

 stated at about 14 cubic feet, but is sometimes, with good coal and careful 

 stoking, much higher. 



The author briefly alludes to the progress of improvement in Cornwall ; 

 the introduction by Woolf of high steam ; the substitution of the plunger 

 pole for the bucket pump, and the application, so recently made by Mr. 

 James Sims, to stamping or crank engines of the arrangements which had 

 been a long time so advantageously in use in pimiping engines. 



The communication is accompanied by a table, exhibiting the weight of 

 water per cubic foot ; the pressure ; the volume and the efficiency of steam 

 from one to ten atmospheres, adapted and corrected from those of Clement 

 and Desormes. It is also accompanied by a method of representing several 

 particulars connected with the load and engine ; by which the relation of 

 these with respect to each other in the same engine, and the ditl'erent condi- 

 tions of other engines, may be at once exhibited to the eye. It may also 

 prove a convenient method of recording facts and calculations in connexion 

 with the Indicator Diagrams. 



" Analysis of a piece of the iron heel post converted by the action of Sea 

 Water into a substance resembling Plumbago." By David Mushet, A. 

 Inst. C. E. 



A piece of the iron heel post of a vessel called the John Bull had been 

 presented last Session by Mr. Borthwick, as a cmious specimen of the eft'ect 

 of salt water in converting iron into a substance resembling plumbago. This 

 substance was of a dark brown colour, and easily cut by a knife, and Mr. 

 Mushet undertook to analyse it ; and the result of this analysis, and the me- 

 thods pursued, aie the subject of this communication. This substance, which 

 it may be convenient to call marine plumbago, on being exposed to a red 

 heat in a crucible, lost about 20 per cent, in weight, and on being exposed to 

 a wliite heat for four hours lost about 60 per cent., and came out a light mass 

 of very brilliant carburet. This shining carburet was then used as a carljou- 

 aceons substance for the reduction of an oxyde of iron, but was less efficacious 

 than the same quantity of the charcoal of wood. From these and other ex- 

 periments, Mr. Mushet considers 100 parts of the so-called marine plumbago 

 to be composed near as follows :— 



Carbonic acid and moisture . 20 

 Protoxyde of iron . . . 35'7 

 Silt, or earthy matter . . 7'2 

 Cwbon . ' . . . . 41-1 



Also, he considers 100 parts of the common black lead (o consist as follows : 

 Carbonic acid and water . 12-5 

 Iron ..... ll'j 

 Earthy matter . . .4-5 

 Carbon 71 '3 



" A theoretical calculation of the Fuel saved by working Steam expansively." 

 By J. W. Lubbock, Hon. M.'inst. C. E. &c. &c. 



.\n equation may be readily formed for the action till the steam is cut oft"; 

 and the steam being then supposed to dilate into a certain volume, tlie varia- 

 tion in this volume gives rise to the quantity of action, whence anotlicr equa- 

 tion may be obtained, and the maximum of the quantity of action produced 

 by cutting oft' the steam determined. The quantity of action thus produced 

 is then compared with that produced in any case without cutting oft' the 

 steam. Now the quantity of heat or fuel expended is proportional to the 

 steam generated in each of the preceding cases, and a proportion, expressing 

 the ratio of the fuel saved to the fuel expended, may be obtained. 



" On the Expansion of Arches." By CJeobge Ren'xie, F.R.S. 



The expansion of solids, which has excited the attention of mathematicians 

 since the investigation of La Hire, in 1688, on a rod of iron, is of particular 

 importance in the construction of bridges, the security of which may be 

 eft'ected by the dilatation and contraction consequent on changes of tcmjiera- 

 tm'e. Periodical motions, referable only to changes of temperature, were ol)- 

 served by Vicat in a stone bridge built over the DordoRne at Souillac, and 

 have frequently been noticed in structures of all kinds. The diiferent expan- 

 sibility of stone and iron has been considered an objection to the use of cast 

 iron pUlars in connexion with stone to support the fronts of buildings ; but 

 the experiments of Mr. Adie of Edinburgh led him to the conclusion, that no 

 danger is to be apprehended from a change of temperature aft'ecting cast iron 

 and sandstone in any great degree, as their expansion, so far as regards build- 

 ings, may be considered the same. 



Arguments from this source were employed agaiust the arches of South- 

 wark Bridge, and the experiments set forth in this communication were 

 undertaken with a view of ascertaining the eft'ect of temperature on these 

 arches. 



Three sets of experiments were made: the first in Jan. 1818, when the 

 main ribs and diagonal braces rested on their centres, and before any of the 

 spandrils and road plates had been put upon them ; the second, in the A\igust 

 and September of tlie same year. The rise was measm-ed by tlie insertion of 

 small wedges to about ;^th of an inch. The third set of experiments was 

 made on the eastern arch. Three thermometers were employed — one hang- 

 ing in the open air, another having the bulb immersed 1 A inch in the iron, 

 and the third hanging amongst the ribs ; these were observed at dift'crent 

 hoiu-s of the day, and the rcsidts recorded. The rise of the arch was ob- 

 served by a fine piece of feathered edged brass, nicely tixed to the rib, which 

 by the rise and fall of tlie arch traversed upon a scale graduated to ^Vr*!' of 

 an inch. The tables contain experiments on nine days, with the teuipcra- 

 tm'cs and rise at every hour of the day. The results, that is, the maximum 

 temperatures and rise, and rise for 1' Fahr. are exhibited in the following 

 fable. 



The mean rise is ^th of an inch for 1° Fahr. 



Mr. Rennie then proceeds to calculate the theoretical rise from the ex- 

 pansion of iron, according to Lavoisier, in an arch of the dimensions of South- 

 wark Bridge, for 50' increase of temperature. 



The eff'ects of changes of temperature were also observed in the stone 

 bridge over the Thames at Staines. After the arches had attained their full 

 settlement, openings were observed in the joints of the parapets immediately 

 over the springing of the arches, and a distortion or sinking of the upper 

 curve of the parapets. A wedge was inserted into some of these openings, 

 and the lowest point of its descent in the month of January marked. The 

 same wedges were carefully inserted every week until May, when they would 

 no longer enter, and the joints became firmly closed. At this period, how- 

 ever, the joints immediately over the crowns of the arches, which hail during 

 the winter been quite close, were now open. From these facts it followed, 

 as a necessary consequence, that in winter the arch contr,icting descended 

 and the spandiil joints opened, and in summer the arch expanding rose and 



