i68 Dr. Brewster on the communication of double 
applied in the direction both of their neutral and depolarising 
axes, forces so great as to break the shoulders of all the 
clamps that were employed. 
Proposition XIV. 
To construct a chromatic dynamometer for measuring the intensity 
of forces. 
In almost every dynamometer, which has hitherto been 
constructed, it is assumed that a steel spring recovers its 
original shape after repeated bendings, and upon this assump- 
tion the scale of the instrument is formed.* The perfect 
elasticity of glass, however, renders it, in this respect, a much 
fitter substance than steel, and though it does not admit of 
such a great change of shape, yet the slightest variations in 
its structure can be rendered visible. 
If a number of narrow and thick plates of glass AB, Fig. 14, 
(PI. IX.) are firmly fixed at each end in brass caps A, B ; then 
if any force is applied to a ring at C in the middle of the plates, 
when the ends A and B are fixed, or if C is fixed, and the 
force applied at the points A, B, the plates of glass will be bent 
in the middle, and the force by which this is produced, will be 
measured by the tints that appear on each side of the black 
space mn. By diminishing the length of the plates, or increasing 
their number, they may be made to resist and to measure any 
degree of force. When the force to be ascertained is small, 
a single plate of glass will enable us to measure, its intensity 
with great exactness. 
* In the article Dynamometer, in the Edinburgh Encyclopedia, Vol. 
VIII. 299, I have described an instrument in which a variable measure of force is 
obtained by raising a metallic cylinder out of a fluid, 
