CHAP, ix.] THE LOCOMOTIVE. 481 



driving-wheels of the carriage, depends, with the same velocity of the 

 piston, on the number of teeth in the wheel and the pinion. But 

 the driving-shaft has another pinion, which works in a second wheel, 

 itself fixed on another driving-shaft parallel to the first, and this last, 

 by a third pinion, communicates its motion to the first cog-wheel. 

 It is of course understood that these two systems work independently. 

 The conductor passes at pleasure from one to the other, by the aid 

 of adapting levers within reach. He can thus vary the velocity of the 

 driving-wheels, for the same work of the steam, in a ratio which 

 varies from the simple to the double (more exactly from sixteen to 

 thirty- nine). 1 



Of late years in England efforts have tended in the direction of 

 effecting rapid transit on tramways by means of compressed air, and 

 of giving up high speed on ordinary roads altogether. The use of 

 the traction-engine for heavy loads is, however, increasing ; that 

 chiefly used, designed by Aveling and Porter, is shown in the annexed 

 woodcut. 



It would appear that the action of the traction-engine on the 

 roads on which it has travelled has given rise to a new employ- 

 ment of steam, for in the steam-roller we have a locomotive the. 

 object of which is to make smooth roads rather than use them. In 

 principle it will be seen the steam-roller is a locomotive with a 

 great development of weight and width of wheel. 



The engine is carried upon four rollers of equal widths, the two 

 hind ones acting as drivers, and the two in front as steering-rollers. 

 These latter cover the space between the two driving-rollers, and are 

 made slightly conical in order that on the ground line the} 7 may run 

 close together while leaving room above their axle for the vertical 



o 

 1 



The mechanical problem of steam locomotion on ordinary roads may perhaps, 

 as we have seen, be considered solved. Can we say from this that the employment 

 of road locomotion will become general ? It is difficult to answer this question, for 

 besides the technical aspect of it, there is the industrial and commercial. This 

 means of transport would have to be really economical ; and this evidently depends 

 on a variety of circumstances in no sense connected with mechanics. In great 

 cities, such as London and Paris, where the requirements of locomotion are so 

 continuous and pressing, road-engines may perhaps be usefully employed, if means 

 could be devised to render it prudent, and to guard against the dangers that would 

 arise every instant in meeting carriages and foot passengers. It is probable that 

 this mode of locomotion will be tried, and perhaps definitely adopted, on some of 

 the tramways. 



I I 



