440 STEPHENSON'S PATENT 



engines which have no fly-wheels are always in pairs, like locomotives, except in 

 some of the smallest vessels. 



The connecting rod being inclined below the piston rod during the back stroke, 

 and above it in the fore stroke, requires a moving joint at the end which takes hold 

 of the cross head ; and the cross head is made spherical at that part, to prevent any 

 lateral strain that might arise during the motion from the crank not being accurately 

 in the line of the piston rod, or the axle at right angles to it. The varying 

 position of the connecting rod also renders the guides for the piston rod necessary to 

 resist the great oblique strain upon it caused by the inclined positions of the con- 

 necting rod tending to force it upwards in the back stroke, and in the opposite 

 direction during the fore stroke. This oblique strain is diminished by increasing 

 the length of the connecting rod ; which is therefore made as long as possible, in 

 order to diminish the friction of the guide blocks, leaving only a small clearance beyond 

 the crank and head of the piston rod at their extreme positions. Other modifications 

 of the plan are also used to preserve the parallel motion of the piston rod, such as a 

 single square bar placed on each side with the edges at top, termed diamond guides, 

 and having sockets on the ends of the cross heads sliding upon them ; but the other 

 plan is found to be most advantageous ; in small stationary engines similar plans 

 are also sometimes adopted. But in stationary and marine engines generally, the 

 motion of the piston is maintained in a straight line by various combinations of 

 rods, forming a parallel motion, which has less friction than guides, and is more 

 convenient in those cases. The strain is also diminished by the piston rod being 

 connected with the end of a beam instead of directly with the connecting rod, which 

 has a much less irregular motion. 



The two cranks are thus made to revolve uniformly by the action of the steam 

 upon the pistons in the cylinders, and move with them the axle and the wheels 

 fixed upon it. The wheels are made to revolve in the same direction that they 

 would turn if the engine were running forward ; and they cannot turn round without 

 either slipping round upon the rails, or rolling forward upon them and moving the 

 engine with them. If the adhesion of the wheels upon the rails is greater than the 

 resistance of the engine to being moved, and the pressure of the steam be sufficient, 

 the wheels will roll forward upon the rails ; and the engine will be propelled, and 

 be able to draw after it a load, the resistance of which is equal to the excess of the 

 adhesion of the wheels upon the rails above what is required for moving the engine 

 itself. The adhesion of the wheels is not always the same ; it is the greatest when 

 the rails are most clean, and are either quite dry or quite wet ; and it is least 

 when the rails are dirty, and greasy with being partially wetted. For this 



