364 PROCEEDINGS OF SECTION H. 
At the outset it is to be stated that there are two entirely 
distinct classes of irregular and undesirable motions in loco- 
motives, the first originating in the actual pressure of steam on 
the pistons, and the second due to the inertia and centrifugal 
action of the moving parts. The first class are quite un- 
affected by any balancing or want of balance, and are felt most 
at low speeds and when full power is being exerted. They con- 
sist of pitching and rolling movements (these terms being used 
in the uusal nautical sense), and are due to the varying pres- 
sure of the cross heads on the guide bars, and in engines with 
inclined cylinders to the vertical resolved part of the steam 
pressure on the cylinder cover. To minimise the pitching we 
should keep the cylinders as nearly horizontal as possible, have 
long connecting rods, a long wheel base, and stiff springs. To 
minimise the rolling we should comply with the first, second, 
and fourth of the preceding conditions, and also prefer inside 
cylinders to outside. There appears also some reason to believe 
that a boiler with its centre high up tends to check rolling. 
The heaviest pitching the writer has observed was in a six- 
coupled-inside cylinder engine, with very sloping cylinders, 
struggling with a_ heavy load up a steep incline, and the 
heaviest ‘rolling: with a four-coupled bogie outside cylinder 
engine, also with steeply-inclined cylinders, under similar con- 
ditions. Of these irregular movements there is nothing more 
to say, having pointed out how they may be reduced to compara- 
tive harmlessness. The irregular movements due to inertia 
and centrifugal acticn of moving parts are, contrary to the pre- 
ceding, felt most at high speeds, when the pressure of steam on 
the pistons is small or non-existent, as when running down- 
grade. They consist in inside-cylinder engines of a fore and 
aft jerking destructive to couplings, and most uncomfortable 
to the men. In outside-cylinder engines this is combined with 
a sideways motion of the front of the engine, which beats the 
flanges of its leading wheels alternately against each rail. In 
addition to this, there is another and very dangerous action 
that may easily pass unnoticed until a certain critical speed is 
reached, when it evidences itself in a most violent and disas- 
trous manner. This is a periodic variation in the pressure of 
the driving wheels on the rails, which at all times means extra 
stress on the rails, and which at a certain speed easily cal- 
culated by the methods of dynamics, becomes so great that at 
a certain point in the revolution the wheel rises off the rail, 
and comes down upon it again lke a huge hammer. This 
actually happened in the case of locomotive 365X some twelve 
years ago. This engine had been rebalanced in accordance 
with an erroneous calculation, and carried 300 lbs. more balance 
weight in each driving wheel than it should. Running freely 
down along a steep erade the critical speed was exceeded : 
