THE STEAM-ENGINE. 539 



the waste steam into the chimney ; 2d, the almost unlimited abstraction of heat 

 from the air passing from the furnace, by arrangement of tubes traversing the 

 boiler ; and, 3d, keeping the cylinders warm, by immersing them in the cham- 

 ber under the chimney. There are many minor details which might be noticed 

 with approbation, but these constitute the main features of the improvements. 



The great original cost, and the heavy expense of keeping the engines used 

 on the railway in repair, have pressed severely on the resources of the under- 

 taking. One of the best-constructed of the later engines costs originally 

 1,500/., and sometimes more. The original cost, however, is far from being 

 the principal source of expense : the wear and tear of these machines, and 

 the occasional fracture of those parts on which the greatest strain has been 

 laid, have greatly exceeded what the directors had anticipated. Although 

 this source of expense must be in part attributed to the engines not having yet 

 attained that state of perfection, in the proportion and adjustment of their parts, 

 of which they are susceptible, and to which experience alone can lead, yet 

 there are some obvious defects which demand attention. 



The heads of the boilers are flat, and formed of iron, similar to the material 

 of the boilers themselves. The tubes which traverse the boiler were, until 

 recently, copper, and so inserted into the flat head or end as to be water-tight. 

 When the boiler was heated, the tubes were found to expand in a greater de- 

 gree than the other parts of the boiler ; which frequently caused them either 

 to be loosened at the extremities, so as to cause leakage, or to bend from want 

 of room for expansion. The necessity of removing and refastening the tubes 

 caused, therefore, a constant expense. 



The fireplace being situated at one end of the boiler, immediately below the 

 mouths of the tubes, a powerful draught of air, passing through the fire, carries 

 with it ashes and cinders, which are driven violently through the tubes, and 

 especially the lower ones, situated near the fuel. These tubes are, by this 

 means, subject to rapid wear, the cinders continually acting upon their interior 

 surface. After a short time it becomes necessary to replace single tubes, ac- 

 cording as they are found to be worn, by new ones ; and it not unfrequently 

 happens, when this is neglected, that tubes burst. After a certain length of 

 time the engines require new tubing. This wear of the tubes might possibly 

 be avoided by constructing the fireplace in a lower position, so as to be more 

 removed from their mouths ; or, still more effectually, by interposing a casing 

 of metal, which might be filled with water, between the fireplace and those 

 tubes which are the most exposed to the cinders and ashes. The unequal 

 expansion of the tubes and boilers appears to be an incurable defect, if the 

 present form of the engine be retained. If the fireplace and chimney could 

 be placed at the same end of the boiler, so that the tubes might be recurved, 

 the unequal expansion would then produce no injurious effect; but it would be 

 difficult to clean the tubes, if they were exposed, as they are at present, to the 

 cinders. The next source of expense arises from the wear of the boiler-heads, 

 which are exposed to the action of the fire. 



A considerable improvement was subsequently introduced into the method 

 of tubing, by substituting brass for copper tubes. I am not aware that the 

 cause of this improvement has been discovered ; but it is certain, whatever be 

 the cause, that brass tubes are subject to considerably slower wear than cop- 

 per ones. 



Since the date to which the preceding observations refer, the locomotive i, 

 engine has undergone several improvements in detail of considerable import- > 

 ance ; among which, the addition of a third pair of wheels deserves to be ( 

 particularly mentioned. An engine supported on three pairs of wheels has > 

 great security in the event of the fracture of any one of the axles the remain- ( 



