102 STEPHENSON'S SECOND LOCOMOTIVE. CHAP. VJ. 



of the permanent road could not always be maintained 

 at the same level, that the wheel at one end of the 

 axle might be depressed into one part of the line which 

 had subsided, whilst the other wheel would be com- 

 paratively elevated ; and, in such a position of the axle 

 and wheels, it was obvious that a rigid communication 

 between the cross head and the wheels was impracticable. 

 Hence it became necessary to form a joint at the top of 

 the piston-rod where it united with the cross head, so 

 as to permit the cross head to preserve complete parallel- 

 ism with the axle of the wheels with which it was in 

 communication . 



In order to obtain that degree of flexibility combined 

 with direct action, which was essential for ensuring 

 power and avoiding needless friction and jars from 

 irregularities in the road, Mr. Stephenson made use of 

 the " ball and socket "joint for effecting a union between 

 the ends of the cross heads where they united with the 

 connecting rods, and between the ends of the connecting 

 rods where they were united with the crank-pins attached 

 to each driving wheel. By this arrangement the paral- 

 lelism between the cross head and the axle was at all 

 times maintained and preserved, without producing any 

 serious jar or friction on any part of the machine. 

 Another important point was, to combine each pair of 

 wheels by means of some simple mechanism instead of 

 by the cogwheels which had formerly been used. And, 

 with this object, Mr. Stephenson began by making in 

 each axle cranks at right angles to each other, with rods 

 communicating horizontally between them. 



A locomotive was accordingly constructed upon this 

 plan in the year 1815, and it was found to answer 

 extremely well. But at that period the mechanical 

 skill of the country was not equal to the task of forging 

 cranked axles of the soundness and strength necessary 

 to stand the jars incident to locomotive work. Mr. 

 Stephenson was accordingly compelled to fall back upon 



