CHAP, ix.] THE LOCOMOTIVE. 477 



constant. The piston E receives on its anterior face the action of the 

 spring, which can be regulated at pleasure. As to the other faces of 

 the two pistons, they are subjected to equal actions proceeding from 

 the pressure of the gas at its entry, actions which thus counteract each 

 other; so that the forces which determine the position of the movable 

 system are on the one hand the tension of the spring, a constant and 

 determined force, and on the other hand, the pressure of the flowing 

 gas; and thus equilibrium cannot occur unless the two forces are 

 equal. If the gas should flow in too great quantity, the pressure 

 increases on the posterior face of the piston N, the spring is overcome, 

 and the movable system advances a little towards the left ; but then 

 the orifices are partly covered and the flow diminishes. If the 

 pressure then becomes too weak at the exit, the spring in its turn 

 prevails, pushes the sheath towards the right, uncovers the orifices, 

 and consequently a greater quantity of air may enter. 1 



V. STEAM- CARRIAGES, on KOAD-LOCOMOTIVES. 



The first steam-carriages w r ere designed for and used upon ordinary 

 roads before the invention of railways. We have seen that they 

 could not succeed. 



Now the reasons of this want of success were manifold ; some arose 

 from the relative imperfection of the steam-engines employed for the 

 purpose, as also the driving machinery, others arose from the very 

 nature of the road on which these carriages were to move. The power 

 of a locomotive has some relation to its weight, although it would be 

 erroneous to believe in this case in the necessity of increasing the 

 weight in order to increase the adhesion. The wheels, and especially 

 the driving wheels, support this always heavy weight, and discharge 

 themselves of it on the road itself at the points where they are in 

 contact with it. Now, however well laid and paved the road may 

 be, the ground yields to the pressure, ruts are formed, and at the end 

 of a short time the engines come to a stand on the road. 



In London in 1862 Bray's locomotives were employed to draw 

 heavy loads on the ordinary macadamized or paved roads, in trenches 



1 For this description the editor is indebted to an account given in Nature, 

 April 2, 1876. 



