THE TEACHING OE MECHANICS BY EXPERIMENT. 
255 
by help of the 
squared paper, 
which allows the 
inclination of the 
cord to be read 
off. 
The principle of 
moments finds 
quantitative illust¬ 
ration in experi¬ 
ments made with a 
steel yard and also 
with a graduated 
beam loaded at 
various points 
along its length 
and supported at 
the ends on two 
spring balances or 
small platform 
balances. Simil¬ 
arly the moments 
of a system of in¬ 
clined forces can 
be made the sub¬ 
ject of experiment 
by means of a 
horizontal board 
or platform 
pivoted on ball 
bearings and 
pulled in various 
directions by cords Fig. 2. 
which pass over 
pulleys and carry loads. 
Turning now to kinetics, I think it is the almost uniform experience 
of teachers that the student has more difficulty in grasping the funda¬ 
mental conceptions in that subject than those of statics, and it is 
therefore all the more necessary to provide ample experimental illus¬ 
tration. Atwood's Machine, already mentioned to-day, is a device of 
great service in assisting a student to grasp the relation of accelera¬ 
tion, force and mass. In most forms of Atwood’s machine the pulley 
at the top is made exceedingly light, and the effect of its inertia is in¬ 
tended to be neglected. I prefer to use a substantial pulley and to 
tell the student what is the equivalent mass, which, moving with the 
cord, would have the same inertia. 
Atwood's machine has been described as a device for diluting 
gravity. Fig. 3 shows another apparatus which also dilutes gravity 
and gives further useful lessons in uniformly accelerated motion. It is 
Apparatus for friction of 
cord on drum . 
