90 



XATCRE 



[May 23, 189; 



engine worked, the lifting of a hea\y mass of pump-rods, contri- 

 buteil to force the use of expansive working. To work without 

 shock, a lai^e initial and gradually diminishing effort was neces- 

 sary". The extraordinar)' economy obtained was due proKibly in 

 (art to the special action of the steam, the Cornish engine being 

 \irtually a comjiound engine, and the admission surface being 

 protected from oxiling to the condenser ; partly to the great 

 effectiveness of a steam-jacket in an engine which worked slowly 

 and with |«uses at the end of the stroke, partly to the small 

 clearance and separate admission and exhaust valves. The les-son 

 engineers should have learned from Cornish experience w as that 

 in restricted conditions high ratios of expansion were economical. 

 In this case, as in others, later engineers generalised too crudely, 

 and concluded that ex|Mnsive working was always economical. 

 A new scientific investigation was required to correct the error. 



Expfrimiiils on Boilers. 



To generate steam power economically the boiler must be 

 efficient, and the engine must be efficient, and the conditions 

 of efficiency of toiler and engine are completely indejiendent. 

 Hence the early method of Watt, in which attention w<as (mid 

 ■only to fuel usetl and water pumped has been replaced by a 

 meth'^d of independent boiler and engine testing. The boiler 

 uses cixil and generates steam : the engine uses .steam and 

 generates power. The economy of the boiler is reckoned on the 

 fuel : that of the engine on the steam. 



Different coals, at any rate the better kinds of coal, do not 

 differ much in absi^ilute calorific value. Used in boiler furnaces, 

 they differ more, (Xirtly from differences of mechanical .iggrega- 

 tion, but chiefly Ijecause the more bituminous coals require a 

 larger air supply for tolerably smokeless combustion than those 

 which consist chiefly of fixed carbon. The greater excess of air 

 involves greater chimney waste. It is to test the commercial 

 calorific value that I'rof. Schroter has established in Munich a 

 heat L-iborator)- where fuels can l>e tested on a lai^e scale and 

 under ordinary practic-al conditions of combustion. The arrange- 

 ments [K-rmit the determination of the exact conditions most 

 suitable for each fuel. 



An enormous number of lioiler trials have been carried out, 

 but most of them are mere individual tests of very little scien- 

 tific value. Engineers have been too much under the impression 

 that the evaiwration deiwnded chiefly on the ty|w or pro]>ortions 

 of the lioiler, or the arrangement of the heating surface. Bui 

 there are no obscure or complicated actions concerned in generat- 

 ing steam. Boilers of all ly|X'S give nearly the .'«iiic results, 

 provided only projier conditions of combustion are secured. 

 They may differ in cost, in durability, in convenience, but in 

 efficiency they differ less than I think is commonly a.ssume<l. 

 The following table shows that Irailers of extremely different 

 types, with very different pro]iortions of heating .surface and ver)' 

 different rates f if combustion, and even with diffeient coals, have 

 all reached eva|iorations of from II to 13 lbs. of w,iter from .and 



K.,li....| 

 Xrali: l.> 

 hcalinK 

 wirfacc. 



HoiUr Trials. 



Coal per <iq, 

 ft. of gr.ilc 

 per hour. 



Lanca.<>hlrc ... 

 Calloway 

 I'ortable ...' 

 Tubular 



M, 



Th'Tneyrroft 



I =36 

 I : 24 

 I : 69 

 I : 46 

 I :3« 

 1 =34 

 I :SO 

 I : 70 

 I • (-.1 



7 ^ 

 22 9 



8-5 



12-8 



IO-8 



8-9 



22 '4 



255 

 77 

 1X0 



.Miiliioiisc J'rials of 1S59. 



The earliest Imilcr trials carried out in a completely satis- 

 farinry «ay were those made by the Snciele Induslriclle of 

 MulhMU«e in 1859. The Society offered a prize to the maker of 

 any lioiler which wnuM evajxiratc 1800 llis. |ier hniir, at 75 lbs. 

 per vjuare inch pressure, and which would eva|>orate 9' I llis. of 



water, from and at 212', i>er jxiund of .Msatian coal of not veiy 

 good quality. With the coal used in these trials, 130 cubic 

 feet of air jwr |x>und of coal are chemically necessiiry lor com- 

 plete comliustion. It w.as found that the reduction of the air 

 supply almost to this limit, and to a |X)int at which there w.is 

 definitely incomplete combustion, reduced the chimney waste 

 and increased the efiiciency of the boiler. In two si^iecial trials, 

 each of a week's duration, the evajxiration was 9 lbs. with 331 

 cubic feet of air, per [xiund, and 9'53 or 6 per cent, more with 

 247 cubic feet. 



The determination of the air supply to a boiler is not 

 altogether an easy operation. .\x\ anemometer was used in 

 .Vlsace, and in suitable conditions it will give approximately 

 accurate results. In recent trials chemical analy.ses of s;impks 

 of the furnace gases have been made, and the amount of air 

 supplied calculated from the percentage of CO5. This method 

 is accurate in principle, but the samples analysed are a very 

 minute fraction of the total chinmey discharge, and the sjimjiles 

 may not lie average samples. In some trials in which thi^ 

 meth'xl has been used, there are discreixmcies in the ratio of tlu- 

 chimney lo.ss and the undetermined lo.ss, chiefly due to radiation, 

 difficult to understand. Neither anemometer nor chemical 

 analysis is suited to serve as a means of regulating the air 

 supply in the ordinary working of a Ixiiler. What is wanted is 

 an instrument as easily reati as a pressure gauge, and giving 

 continuous indications. 



The Dasymeler, 



The d.isymeter, invented by Messrs. Siegert and Durr, of 

 Munich, is a tine balance in an enclosed case through which a 

 current of the furnace gases is drawn. .\t one end of the 

 balance is a glass glolie of large displacement, at the other a 

 brass weight. .\ny change of density of the medium in the 

 chamber disturbs the balance. .A finger on the balance utoving 

 over a graduated .scale gives the amount of the alteration of 

 density. .\n air injector draws the furnace gas from the flues. 

 and it is filtered before entering the balance case. .\n ingenious 

 mercurial com|K'nsator coimterUilances any elVect tlue to change 

 of temperature or barometric pressure. 



The dasymeter is usually combined with a draught gauge, and 

 an air thermometer or pyrometer in the flue is required if the 

 amount of w.aste heat is to be calculated. The dasymeter I 

 requires, initially, exceedingly delicate adjustment, and its indica- 

 tions must be checked fr<im time to time by a liunte's burette. 

 It is set to read zero with pure air, and then any increase "f 

 ilensity due to C(Jj is read a-s a percentage on the sen. 

 When in adjustment, it is as easy to read the percentage "i 

 COo in the furnace gases as to read the pressure on a 

 pres-sure gauge. When the das)nneter is fitted to a boiler, 

 the stoker has directions to adjust the supply of air so that the 

 furnaces gases have almut 12 percent, of CfX. With practice 

 he learns what alterations of the dam|ier or fire-door, or thick- 

 ness of fuel on the grate, are necessary, or whether an alteration 

 of grate area is desirable, .\fter a little time the percentage of 

 CO.j can lie kept very constant. 



Isliii-nvoifs E.xfcriiiiciils on Marine linginii. 



.Alxiut the year i860, Mr. Isherwood, Chief ICngineer of the 

 United States Navy, began a series of systematic tests of engines 

 and lioilers on a very large scale, and with resources only avail- 

 able in a (lovernnient establishment. The trials were made 

 with skill and determination, and the substantial accuracy of the 

 results, startling as they were, h.as never been iiuestioned. 



.All Isherwood's trials of large marine engmes showed that 

 when expansion was extended beyond exceedingly small limit.s, 

 it caused not an economy, but a waste. In his second volume 

 he sums up his results as proving that when cutoff is earlier 

 than o'6, or |x'rhaps even 07 of the stroke, the c<insuniption of' 

 steam reckoned o[i the work done is increii.sed. Curiously 

 enough, this leil him to attack the compound engine. Krom the' 

 quantities in the table of exix-rimenls, he says, " it will be seen 

 how useless in piint of economic gain is the prejxislerous ar- 

 rangement of steam engine known as the doublecylmder, Woolf, 

 or llornlilower engine. . . . ()p|K)sed to these facts, the declara- 

 tions of inleresleil patentees and engine builders must be classed 

 in value with those sel forth by (juacks in advertiscnnents of their 

 nostrums." This is frcmi a paper dated 1865, and it is curious, 

 liecau.se Isherwixxl generally saw clearly enough the danger 

 of drawing sweeping conclusions from narrow experimental 

 premises. 



The pro|)er lesson from Ishcrwootl's results was merely that 



NO. 1334, VOL. 52] 



