THE STEAM-ENGINE. 



537 



smaller diameter than three inches were not used. In engines since con- 

 structed, the number of tubes vary from ninety to one hundred and twenty, the 

 diameter being reduced to two inches or less ; and in some instances tubes 

 have been introduced, even to the number of one hundred and fifty, of one and 

 a half inch diameter. 



Since the period at which this railway was opened for the actual purposes 

 of transport, the locomotive engines have been in a state of progressive im- 

 provement. Scarcely a month has passed without suggesting some change 

 in the details, by which fuel might be economized, the production of steam 

 rendered more rapid, the wear of the engine rendered slower, the proportionate 

 strength of the different parts improved, or some other desirable end obtained. 



Engines constructed in the form of the Rocket, were subject to two principal 

 defects. The cylinders, being placed outside the engine, were exposed to the 

 cold of the atmosphere, which produced a waste of heat more or less consider- 

 able by condensation. The points at which the power of the steam to turn the 

 wheels was applied, being at the extremities of the axle and on the exterior of 

 the wheel, a considerable strain was produced, owing to the distance of the 

 point of application of the power from the centre of resistance. If it were 

 possible that the impelling power could act in drawing the train at all times 

 with equal energy to both sides of the engine, then no injurious strain would 

 be produced ; but from the relative position of the points on the opposite wheels 

 to which it was necessary to attach the connecting rods, it was inevitable that, 

 at the moment when one of the pistons exerts its full power in driving the 

 wheel, the other piston must be altogether inactive. The impelling power, 

 therefore, at alternate moments of time, acted on opposite wheels, and on each 

 of them at the greatest possible distance from the centre of the axle. 



The next step in the improvement of the machine was made with a view to 

 remove these two defects. The cylinders were transferred from the exterior 

 of the engine to the interior of the casing called the smoke-box, B, fig. 64, 



Fig. 64. 



which supports the chimney, and which receives the heated air issuing from 



the tubes which traverse the boiler. Thus placed, the cylinders are always 



ij maintained as hot as the air which issues from the flues, and all condensation 



I of steam by their exposure is prevented. The piston-rods are likewise brought 



