LOCOMOTIVE ENGINE. 413 



cut quite through with a chisel and then turned inwards so as to detach it from the 

 tube, which can then be driven out. 



By causing all the flame and heated air to pass through a great number of small 

 tubes surrounded by the water, a very great and rapid means of heating the water 

 is obtained, as a very large heated surface is thus exposed to the water. The first 

 locomotive engines had merely a large flue passing from the fireplace to the chimney; 

 it was bent round at the end and returned again to the back, the chimney being 

 placed at the same end as the fireplace ; the fire was contained in the commence- 

 ment of the flue, which was made larger for the purpose. This is the general prin- 

 ciple of the construction of the boilers for stationary engines where the size and 

 weight of the boiler are not of so much importance, and the flues can be made large 

 enough to get a sufficient area of heated surface in contact with the water. But as 

 in a locomotive engine all the machinery has to be moved at a great velocity, the 

 size and weight of the boiler are obliged to be diminished very much, and some other 

 means has to be adopted to obtain the requisite heating surface. 



The Rocket engine made by Mr. R. Stephenson, which was the engine that gained 

 the prize for the best locomotive, at the opening of the Liverpool and Manchester 

 Railway in 1829, was the first engine made with tubes in this country*. 



The former locomotives with only a flue through the boiler, had never been able 

 to travel faster than about eight miles an hour, as they had not sufficient heating- 

 surface in the boiler to generate the steam for supplying the cylinders more rapidly ; 

 the speed attainable by a locomotive being limited only by the quantity of steam that 

 can be generated in a given time. The introduction of tubes into the boiler is one of 

 the greatest improvements that has been made in the construction of locomotives, and 

 was the cause of the superiority of the Rocket engine to those that competed with it, 

 and to all the former engines. The velocity it attained at the competition trial was 

 29 miles an hour, and the average 14^ miles an hour. 



The tubes of the Rocket engine were 3 inches in diameter, and only twenty-four in 

 number ; in the engines made subsequently, the size was reduced and the number of 

 them doubled and trebled, by which means the heating surface was very much 

 increased and with it the power of the engine. The smaller the tubes are, the 

 greater is the heating surface obtained, as small circles have a much larger 

 circumference in proportion to their area than large ones ; but when the tubes are 

 diminished in size, the total area of passage through them from the fire-box to the 

 chimney is also diminished; and consequently if the diameter of the tubes were much 



* It appears that the merit of the first invention of a boiler with tubes is due to a French engineer, 

 M. Seguin, who had a patent for it in 1828 ; although the application of the principle in the Rocket 

 engine was undoubtedly an independent invention. 



