THE TEACHING OF MECHANICS BY EXPEEIMENT. 261 
lecture, and is only mentioned as an illustration of the kind of use to 
which a mounted motor forming a dynamometer may be put. 
There is no time to speak in detail of experiments on hydro¬ 
mechanics or on heat. In hydro-mechanics one makes laboratory 
experiments on the friction of fluids in pipes, on the discharge of 
fluids through orifices and over weirs, on the pressure of jets against 
fixed surfaces. 
Many instructive experiments may be arranged in the subject of 
elasticity, in addition to the experiments which are commonly per¬ 
formed by means of a large testing machine. The measurement of 
Young’s Modulus of elasticity in a stretched wire may be easily 
enough effected by hanging up a long wire and measuring its 
extension directly by means of a scale, which is loaded with weights. 
Or we may use a shorter piece of wire and measure the extension by 
letting the wire to be stretched be hung alongside of another wire 
which is kept taut by a considerable constant load, the two hanging 
from the same support. Let a little platform be clamped to each 
and let a small three-legged stool rest with two feet on one of these 
platforms and the other foot on the other platform, and let the stool 
carry a mirror, the deflection of which may be read by means of a 
telescope. The deflection of the mirror takes place of course in altitude 
not in azimuth. We have thus a means of seeing by what small dis¬ 
tance the wire is extended when each successive increment of load 
takes place. I make this piece of apparatus self-contained, the read¬ 
ing telescope, the wires and all being attached to one frame. 
Fig. 7 shows a self-contained apparatus for exhibiting the elastic 
Fig. 7. Apparatus for Elastic Bending of Mods. 
