470 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



III. THE PRINCIPAL TYPES OF LOCOMOTIVES. 



If the locomotive lias a special character which distinguishes it 

 from other steam-engines, such as the fixed industrial engines, or the 

 movable engines for navigation, it does not follow that it constitutes 

 a single and uniform type. It is a genus, but this genus comprises 

 numerous species and varieties. 



These species, of which I can only describe the principal, have 

 been successively formed for the many and increasing requirements 

 of the various kinds of transport. Locomotives may be primarily 

 divided into two very distinct types : 



The passenger-engines, solely destined to carry rapidly trains of 

 no great weight. (Express service.) 



The goods-engines, specially set apart for moving with moderate 

 speed very heavy loads. (Slow service.) 



Naturally a third type, intermediate between the two first, partici- 

 pating in their mean qualities, must have arisen. These are : 



Mixed locomotives, employed to draw trains with passenger car- 

 riages and goods waggons together, or perhaps capable of being used 

 either for fast or slow trains. 



Besides these three principal types other forms of locomotives 

 have been constructed for special purposes. We will pass in review 

 some examples of each of them. 



First the express passenger-engine par excellence (Fig. 317). 

 This is Crampton's locomotive, characterized by the large diameter of 

 its two driving-wheels, and the short stroke of the piston ; two condi- 

 tions which, joined to a high vaporising power, make it the race-horse 

 of the iron way. For the thirty-five years that this excellent engine has 

 been tried, it has not ceased to respond to all the demands of the 

 service. It has great stability, arising from the lowness of the general 

 centre of gravity, and the interval between its axle-trees. Of a 

 mean weight of thirty tons, it will draw a train of twelve or sixteen 

 carriages of 100 to 130 tons, with a velocity, including stoppages, 

 of thirty-seven miles an hour. 



A Crampton, without its tender, costs 2600. 

 The engines of Macconnell, Buddicomb, Sturrock, and Stephen- 

 son's three-cylinder, are also good express engines. The third 



