132 SEVENTH REPORT—1837. 
On the Resistance to Railway Trains. By Dr. LARDNER*. 
The object of this communication was to direct attention to the 
principles which ought to be preserved in determining railway con- 
stants, and especially to the importance of taking into the account the 
moment of inertion of the wheels, which had been generally omitted. 
Dr. Lardner detailed generally the various resistances to which the 
motion of a train is subject; and, having stated his objection to the 
use of the dyanometer, proceeded to explain the method which he 
would fecommend. The principle is as follows: “Let an engine be 
loaded with as heavy a train as it is capable of drawing at a very slow 
and uniform speed, having its steam-valve fully open, and no steam 
blowing off at the safety-valve. Let care also be taken that the 
diameter of the steam-pipe, from the boiler to the cylinder, shall bear 
a considerable ratio to the diameters of the cylinders. Under such 
circumstances we may, without sensible error, consider the pressure 
of steam in the boiler and the cylinders to be the same; and if no 
steam blow off from the safety-valve, the indication of the lever will 
be a true measure of the pressure of the steam upon the pistons of the 
cylinders. This pressure is transmitted to the cranks, the mean 
leverage of which being known, the amount of force transmitted to the 
point where the driving wheels rest upon the rails is a matter of casy 
calculation. This will evidently constitute the gross amount of the 
tractive force exerted by the engine; and this foree may be con- 
sidered as expended in moving the train and the engine, and will be 
the tractive force sought.”+ 
The important details of this communication will be found in the 
Railway Magazine as already referred to. 
A Flexible Suspension Bridge. By W. J. Curtis. 
- The peculiar feature of this bridge is the absence of a main chain ; 
each point is sustained by four forces, viz. two bars carried over each 
pier. 
Onan Instrument for ascertaining the Focal Length of Spectacles. By 
Joun Isaac Hawkins. 
Mr. Hawkins mentioned some facts respecting the differences in the 
distances betwixt the eyes of different individuals, and the focal 
distance of the right and left eye. In one extreme case this differ- 
ence was more than 30 inches, the focal distance of one eye being 36, 
and of the other only 3 inches. 
* For an account of this communication and the calculations, see Railway Maga- 
zine, November, 1837, 
t Ibid. 
