274 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Septembeb, 



niimlx'r of lbs. of water wliioh one lb. of coal will evaporate from 



Sl'i', may be calculated as follows : — 



Let 1' be tbe total quantity of coal consumed, then the work 



done liy P will be to raise \V' + w of water from t to 212°, and to 



evajxirate t" — c'/ from 212^ 



Let ;// be the weight of coal required to raise W + w to 212°, from t 

 „ , evaporate t/i — e^ from 212° 



„ „ „ W + w from 212° 



n 



Then = = E, the evaporating power. 



Now V z^ m A- 



p; 



212 - t 

 I 



m 

 n 



„ p w - eq 



But - = TTT-r— ; 



t i c-eq \ _ 



' \SvT~wl - 



WV + w 



iyV + w) (212 -t) + iu'-eq )l 

 PI 



212 -< 



V) — eq 



= E. 



Introducing the values from which the mean temperature t was 

 obtained (first formula), we have eventually — 



(/ + 212 - t') w + (212 - t") W - leg 



P/ 



= E 



in which W is the weight of water in the boiler ; 



«' tlie weight of water drawn from the tanks; 



t' the mean temperature of water in the tanks ; 



t" the corrected initial temperature of water in the boiler. 

 In the preceding formuhe, the latent heat of steam has been 

 taken at 1000, the number generally used in this country; 

 hut after all the calculations had been made on this subject from 

 the e.\]>erimentsby Messrs. Wilson and Kingsbury, and the results 

 >ient in to tlic Admiralty, Regnault's excellent memoir on the 

 '•Latent Heat of Steam" was published. It became necessary, there- 

 fore, to use these new results in tlie future experiments.' These, 

 so far as they apjdy to the present inquiry, are reduced in the fol- 

 lowing table. 



It also became desirable to introduce new corrections, which the 

 progress of the infjuiry showed to be needful. Tlius, Mr. Phil- 

 lips's careful experiments determined the alteration in tlie capacity 

 of tlie boiler at different temperatures, and correction was in future 

 made for this difference. The alteration in tlie capacity of the 

 measuring tanks was also estimated, whenever tlie temperature 

 differed 2° fr(mi that at which they were gauged. Another cause 

 of error, for which allowance should be made, is any difference 

 which may exist between tlie initial and final temperature at the 

 beginning and close of the experiment. This difterence being 

 known by observation, the correction may be applied from the 

 table of expansion of the water in the boiler, given. Introducing 

 these new corrections into the experiments for ascertaining the 



co-efficient of the heating power of the wood, the following are 

 the formuhe used by Mr. Phillips : — 



{W Jrw- w) {l+t) + wt'-]- {w' - w) t" 



p^ _ h. 



In which W is the water let down from the tanks during the 

 experiment. 



«• = The weight of water found in the boilers at commence- 

 ment of experiment. 



w' = The weight of water in boiler at close of experiment. 



/ = Co-efficient of the latent heat of steam. 



t = Quantity of heat necessary to raise the water in tanks 

 from its mean temperature to that at which it is evaporated. 



t' = Quantity of heat necessary to raise the water in the boiler 

 from the initial to the final temperature. 



f" = Quantity of heat necessary to raise water at the tempera- 

 ture of tanks to the final temperature of water in the boiler. 



P = Weight of combustibles consumed during experiment. 



E = The co-eflicient of the heating powers of wood. 



But when the initial is lower than the final temperature, the 

 formula becomes — 



(W + w - w') I -\- Wt + wt' + (w'- w) tf" 

 PI 



E. 



All the terms retaining their original value except the last, in 

 which t" is replaced by t'" (or the heat necessary to raise the final 

 temperature to that at which the water was expanded), and must 

 be regarded as having a negative value, while f becomes positive. 

 If now q is the weight of wood used in lighting the fire, the 

 formula for estimating the evaporative power of the coal will be 



(W - Eq + w- w') l + {W-\-w- w') t + wt' -\- (»/ - w) t" 



And 



PI 



(W - E 7 -^ TO - ic') < -)- Wt 4- wt' -(-(«/- v) t'" __ 



PI ~ 



E' 



E'. 



As the experiments are strictly comparative, and under like 

 conditions, the want of the other corrections, to which we have 

 alluded above, will not be felt in examining the results; while 

 their execution would have introduced a refinement into the ex- 

 periments which never could be obtained in practice, and which, 

 in fact, would be useless and unwarrantable while, as previously 

 remarked, the errors of observation in all such approsdmative ex- 

 periments remain so large. 



The only omitted correction which in appearance might be sup- 

 posed necessary for practical purposes, is tliat for the hygroscopic 

 condition of the fuel. Had wood been employed, this must have 

 been done ; but the hygroscopic nature of coal is very much less 

 than that of wood. 'Phe latter contains -5- its own weight of hygro- 

 scopic water ; and the heat necessary for the evaporation of this 

 quantity might be shown by a simple calculation to be nearly equal 

 to 22 per cent, of the total heat obtained by the combustion of the 

 wood. The hygroscopic water in coal is however very small, as 

 will be seen liy the following determinations of some of the 

 Welsh specimens experimented upon : — 



Hygroscopic water. 



Graigola Coal 



Anlhra(-ite 



Oiclcastle 



Ward's Fiery Vein 



l\Iynydd Newydd 



Pentrepnth 



Pcntreftlin 



1-06 per cent. 



2-44 



0-74 



1-27 



0-67 „ 



0-78 „ 



0-70 



Had we introduced corrections for these small quantities, prac- 

 tice would have been misled ; because the coals will rarely reach 

 a vessel in the dry state that they did in the jirescnt case, when 

 they were packed in hogsheads and ke])t under cover. — It was 

 found unnecessary to correct for any inflammable gases flying up 

 the chimney, because repeated analyses of the chimney gases 

 proved them not to contain any combustible constituent; the 

 only products ever found being cai^bonic acid, sulphurous acid, 

 oxygen, and nitrogen. The quantity of free oxygen in the 

 chimney varied from j to ^ of tlie oxygen which combined with 

 the fuel ; in other words, nearly twice the quantity of air passes 

 through the fire than that which is strictly necessary by theory. 



M'ith regard to the selection of the coals for trial, we have to 

 refer to Mr. 'Wilson's letter. This letter gives the information 

 obtained in a tout made by Professor AMlson for tlie purpose of 

 ascertaining the best coals fitted for trial in the South Wales coal 

 district, and the ports from which they can conveniently be 

 ship])ed. This district was selected because the -varying charac- 

 ter of the coals, from the bituminous to the anthracitic, ofl^ered 

 those which were most likely to combine the qualities desired for 



