338 
MINUTES OF PKOCEEDINGS OF 
or cart to be fitted with springs. The body, with its load, will not have to 
be raised to the height of each obstacle; for, as the wheels surmount 
it, the springs will yield to some extent, consequent upon their pliancy 
and the vis inertia of the weight above them. In this manner it arrives 
that much less exertion is required to make a wagon with springs pass 
over an obstacle of given height, than to make the same wagon without 
springs pass it. In other words, the draught of the spring wagon is 
the easier. Springs have also the advantage of saving the wagon and 
its load. They do this by lessening the shocks due to the badness of 
the road, acting not only by reducing the height due to an obstacle 
through which the body must rise and fall, but also by rendering the 
blow less severe; in fact, they convert what would otherwise be a blow 
into a pressure. 
On the fore carriage of a wagon, springs are particularly useful; as 
they allow of some yield when, by reason of the unevenness of the 
ground, extra pressure is thrown upon any part in particular of the 
bearing surfaces. 
Two comparatively slight disadvantages accompany the use of springs. 
One is, that the body is not so perfectly rigid over its axles as if without 
springs—some slight lateral motion of the body and load above the 
axles being a necessary consequence of the nature of the connections; 
the other is, that the use of springs necessitates the bottom of the 
wagon being rather higher than it otherwise need be. Both these 
points affect the stability, and the latter also convenience of loading. 
It may also be urged that the addition of springs renders the con¬ 
struction of a cart or wagon more complicated and delicate. No doubt, 
to some extent, this is the case; but the springs used in the service— 
viz., the semi-elliptical—are simple in construction and easily repaired. 
Springs on a wagon or cart should be of such strength that, with the 
full load and the wagon at rest, they should not yield more than half 
the distance (measured along the radius of the spring at the centre, or 
over the point of attachment) through which they would have to bend 
to become straight. 
The strength of a steel spring is proportional, to the number of leaves 
and to the width and thickness of each leaf, while it is inversely pro¬ 
portional to the span. Founded, no doubt, upon this, and upon the 
results of experiments, the following formulae, which give very good 
approximations to the truth, were given in the “ Engineer 33 of 
January 12, 1872 :— 
Let s — span or length in inches of a spring, 
h — breadth in inches of a leaf, 
t = thickness in sixteenths of an inch of a leaf, 
n = number of leaves, 
c = a constant. 
then, the working strength of the spring in tons = 
l x f x 
s x c 
where 
s 6 x c 
c = 11*3 ) and the elasticity or deflection per ton of load = £ x f x n > 
where c = *105. 
