566 
THE SLIP OF THE DRIVING-WHEEL OF A LOCOMOTIVE ENGINE. 
the same with those (59.) which determine whether there be a jump or not, which agrees 
with an observation in the preceding article, to the effect, that as the wheel must cease 
to bite upon the rail before it can jump, it must always slip before it can jump. When 
the conditions of slipping obtain, one of the wheels always biting when the other is 
slipping, and the slips of the two wheels alternating, it is evident that the engine will 
be impelled forwards, at certain periods of each revolution, by one wheel only, and 
at others, by the other wheel only ; and that this is true irrespective of the action of 
the two pistons on the crank, and would be true if the steam were thrown off. Such 
alternate propulsions on the two sides of the train cannot but communicate alternate 
oscillations to the buffer-springs, the intervals between which will not be the same as 
those between the propulsions ; but they may so synchronize with a series of propul- 
sions as that the amplitude of each oscillation may be increased by them until the 
train attains that fish-tail motion with which railway travellers are familiar. It is 
obvious that the results shown here to follow from a displacement of the centres of 
gravity of the driving-wheels, cannot fail also to be produced by the alternate action 
of the connecting rods at the most favourable driving points of the crank and at the 
dead points*, and that the operation of these two causes may tend to neutralize or 
may exaggerate one onother. It is not the object of this paper to discuss the ques- 
tion under this point of view. 
* A slip of the wheel may thus be, and probably is, produced at each revolution. 
Wandsworth, Feh. 28, 1851, 
