CHAP, ix.] THE LOCOMOTIVE. 473 



to 200 tons ; all these elements, it may be seen, are comprised in the 

 corresponding elements of the extreme types. 



Then comes that class of locomotives, sometimes economical and of 

 small relative power, sometimes costly and complicated, but possessing 

 a force of traction which makes them capable of drawing the heaviest 

 loads in damp and rainy seasons, and of ascending the heavy inclines 

 now adopted on a large number of new lines. These last machines, 

 of which Fig. 319 represents a model, are called mountain locomotives, 

 or engines for gradients. It would be necessary for completeness to 

 multiply discussions and figures, to mention the pilot-engines, which 

 give warning or help those drawing too heavy a load, the extra engines 

 sent out in cases of accident, besides the types on foreign lines, the 

 locomotives of German and American railways, heated with wood, 

 whose pointed buffers, rail guards, and chimneys widened at the top, 

 give them so original an appearance. But details so complete and 

 circumstantial would exceed the scope of this work. 



IV. COMPHESSED-AlR LOCOMOTIVES. 



Before we quit the subject of locomotives and railways, there is 

 another kind of engine to be referred to which we introduce in this 

 place in order that the action of steam may be compared with that of 

 another gas under pressure namely, compressed air. 



The boring of a tunnel of any importance presents difficulties of 

 various kinds, among which may .be mentioned the clearing away 

 of the rubbish arising from the excavation of the gallery, whenever 

 that reaches any considerable length. 



Thus in the St. Gothard Tunnel the work was begun from two 

 points, Airolo and Gceschenen, the two extremities of the future 

 tunnel. The advance of the gallery, which is pushed on with activity, 

 produces about 400 cubic metres of rubbish a day at each of the two 

 faces of attack. To carry away this mass of rubbish, which is thrown 

 regularly into trucks running on rails, it is impossible to employ 

 steam locomotives as the cid de sac nature of the galleries prevents 

 effectual ventilation. The high price of horses and the large number 

 required prevent their use. The idea suggested itself of making use 

 of compressed air locomotives. We have already shown how 



