20 The N.Z. Journal of Science and Technology. [Jan. 
big end as revolving and the remainder of the mass (little end -f- J of big 
end) as reciprocating. 
These forces cause, or tend to cause, certain periodic disturbances 
which resolve into (1) a fore-and-aft reciprocation of the engine parallel 
to the direction of motion, called surging; (2) a transverse horizontal 
swaying about a vertical axis through the centre of gravity, called 
nosing; (3) a cyclical variation of the wheel-pressure on the rail, called 
hammerblow. 
When, but not until, this hammerblow exceeds the static weight on 
the rail the whole engine may be vibrated or swayed in a vertical direction, 
but under normal conditions this dangerous point is not reached with 
good balancing. 
Fig. 1. . 
The quantitative treatment of the disturbances is as follows :— 
(1.) The surging - force is the instantaneous sum of the horizontal 
components of the inertia forces ofj-the revolving and recipro¬ 
cating parts, including the balance-weights. 
(2.) The nosing-couple is the instantaneous sum of the moments of 
these horizontal components about the longitudinal central 
vertical plane of the engine. 
(3.) The hammerblow is computed for each wheel by dividing the sum 
of the moments of the vertical components of the inertia forces 
about the vertical plane through the contact of the other wheel 
with the rail, by the transverse distance between the two contact 
planes. 
