262 
THE TEACHING OF MECHANICS BY EXPERIMENT. 
bending of rods. The steel rod to be experimented upon is supported 
by two knife-edges and is loaded at its ends, at two points equally 
distant from the two edges. If the loads at these two points are 
equal then the part of the beam which lies between the two knife- 
edges is subjected to a uniform bending moment; it consequently 
bends into a circular arc, and the amount of rise in the centre is read 
by means of a reading microscope. The beam can also be loaded in 
the centre in the ordinary way. A small mirror fixed to a vice which 
clamps on any portion of the beam serves to measure the angle of 
slope by means of a reading telescope and reflected scale. 
Fig. 8 shows a self-contained 
apparatus for the purpose of experi¬ 
menting on the torsional rigidity of 
wires. A wire is suspended from a 
fixed support at the top, and hangs 
down through the tubular stem of the 
apparatus. Hanging from it is a weight 
in the form of a turned brass drum. A 
cord is passed round the drum and 
carried over pulleys that are fixed along¬ 
side and are as free of friction as 
possible. By applying equal weights 
to the two ends of the cord the wire is 
twisted by a pure couple. Its torsion 
is read by means of a pointer running 
over a circular divided card. Static 
experiments are made in the first in¬ 
stance by applying various amounts of 
equal load to two ends of the cord, and 
after these static experiments the cord 
is removed and the brass drum is used 
as a torsional oscillator. The drum is 
heavy and its moment of inertia can be 
readily calculated from its known 
dimensions and weight. The student 
observes the period of torsional oscil¬ 
lation and so obtains data to work out 
the modulus of rigidity and to compare 
the kinetic with the static modulus. 
It would be easy to cite further 
examples, but enough has been said to Fig , 8 . ^Apparatus for 
show how we may proceed in using Torsion tests. 
experiment as an aid to the 
teaching of mechanics. I do not know whether, from the]point of 
view of the higher flights of mathematical mechanics, experiments 
may not, perhaps, be a mistake. It is possible that a mathematical 
genius of sufficient capacity might find that any appeal to practical 
matters or real things was only a clog on his wings. But for the 
