546 



MECHANICAL ENGINEERING, THE PROGRESS OF. 



efficiency. The Great Eastern, 680 feet long, 

 of 83 feet beam, and measuring 25,000 tons 

 displacement, still remains the largest ship yet 

 built ; but steamers are under construction for 

 transatlantic lines 600 feet long, of over 50 

 feet beam, and fitted with engines of 10,000 

 indicated horse-power. A speed of twenty 

 iniK-s an hour in good weather throughout the 

 voyage, making the distance from land to land 

 in less than a week, may be expected soon to 

 become usual. Double hulls and transverse 

 bulk-heads will make these great vessels safe 

 (.-vcn against the shock of collision with an ice- 

 berg. 



urn-pressure has gradually and steadily 

 men since the time of Watt, when seven pounds 

 half an atmosphere was usual. To-day 

 six atmospheres (75 pounds per square inch) 

 is as usual, and seven atmospheres (90 pounds) is 

 often adopted. Such pressures have compelled 

 the general introduction of the simplest form 

 of steam-boiler the cylindrical tubular boiler, 

 with large flues beneath the tubes, in which 

 the furnaces are formed. Strength of flues is 

 obtained by the use of heavy plates, sometimes 

 flanged at the girth-seams. " Mild " steel is 

 here slowly displacing iron. 



In ordinary practice increase of steam-press- 

 ure with correspondingly increased expansion 

 gives, roughly stated, a decreased steam con- 

 sumption, about in the ratio of the square root 

 of the pressure. This seems true in recent ma- 

 rine engineering; during the past ten years 

 steam-pressure has risen from four and a half 

 to six atmospheres (50 to 75 pounds by gauge), 

 and the consumption of fuel per hour and 

 per horse-power has decreased from 2 to 1'8 

 pounds. Incidentally the area of heating sur- 

 face has decreased from 4 to 4 square feet per 

 indicated horse-power, that is to say remaining, 

 as formerly, nearly 2 square feet per pound of 

 coal burned per horse-power per hour ; where, 

 as in some cases, pressures of 100 and 125 

 pounds are adopted (seven to ten atmospheres, 

 nearly), somewhat further gain may be ex- 

 pected. 



Increased pressure has been accompanied by 

 increased speed of piston from 300 to 500 

 feet per minute and both causes have com- 

 bined to reduce greatly the size and weight of 

 engines. Formerly 500 pounds per indicated 

 horse-power was a common figure ; to-day one 

 half that weight is often noted, and in special 

 cases in which, as in torpedo-boats, economy 

 is not important, one fifth and even one eighth 

 those weights have been reached. 



Surface condensation is almost exclusively 

 adopted, but the area of cooling surface is be- 

 coming less and less, and at the pressure soon 

 likely to become general, the production of a 

 vacuum may possibly cease to be desirable, as 

 it is already known to be with unjacketed cyl- 

 inders; and the non-condensing engine may yet 

 displace the condensing engine at sea as it has 

 on land, and on the Western rivers where this 

 comparison was earlier made, and where the 



evil effects of cylinder condensation were ear- 

 lier perceived. A still for condensing exhaust 

 and waste steam into feed-water has already 

 been used, and it must remain in use in all salt- 

 water navigation. 



Among the most interesting events of the 

 years 1880-'81 have been the trials of the 

 steam-yachts Anthracite and Leila. The first 

 is a small vessel, 86 feet long, 16 feet beam, 

 and 9 feet draught, fitted with a three-cylinder 

 compound engine, and carrying 300 pounds 

 steam (twenty atmospheres, nearly) and up- 

 ward. Trials in London show these engines 

 to have required but Impounds of coal and 

 17'8 pounds of steam per hour and per horse- 

 power. Cylinder condensation amounted to 

 30 per cent in the first cylinder, and of this 

 nearly three fourths was re-evaporated before 

 discharge from the third cylinder. The same 

 engines tested in this country required 21'6 

 pounds of steam per hour and per horse- 

 power, the cylinder condensation becoming 

 over 50 per cent, of which four fifths was 

 re-evaporated before reaching the condenser, 

 the difference being probably due to a varia- 

 tion in the efficiency of the steam-jackets and 

 in speed of engines. This little yacht is the 

 smallest that ever crossed the Atlantic. The 

 trial of the Leila, under the orders of the 

 United States Navy Department, was even 

 more instructive than that of the Anthracite. 

 The Leila is a Herreshoff yacht, 100 feet long, 

 12 feet beam, and measuring 37 tons. With 

 a " coil " boiler, steam at 120 pounds at the 

 steam-chest (nine atmospheres), and driving 

 the boat fifteen knots an hour (seventeen miles), 

 the engines developed 150 horse-power, using 

 but 16 - 4 pounds of steam per hour per horse- 

 power. The cylinder condensation amounted 

 to but 10 per cent. 



An important deduction from the results of 

 the trial of the Anthracite and the Leila is, 

 that efficiency has little relation to size of en- 

 gine when protection against cylinder conden- 

 sation is secured. Some of the best work Las 

 been done, where non-condensing engines have 

 been compared, by small portable engines. 

 Steam-engines of five thousand horse-power 

 are equaled in economy by engines of one 

 fiftieth that power. A large difference in 

 magnitude seems more than compensated by 

 a moderate difference in steam-pressure. The 

 highest pressure may prove least economical 

 when the engineer neglects to provide against 

 loss by cylinder condensation. It is found 

 more and more necessary to discover some 

 means of making the interior surfaces of steam 

 cylinders of non-conducting material. That 

 accomplished, the cost of power, in quantity 

 of steam used, will be reduced from ten to fifty 

 and more per cent, according to the kind of 

 engine considered. Until that is done, super- 

 heating, steam-jacketing, and high speeds of 

 piston must be relied upon to give high effi- 

 ciency ; but only perfectly adiabatic expansion 

 can give maximum economy of steam. The 



