170 Dr. Brewster on the communication of double 
When the fusible metal assumed a settled state, I was sur- 
prised to observe, that the tint over its surface CD, Fig. 16, 
(PI. X.) was not uniform, but had a curved black space mno , 
which inclosed a faint tint belonging to a compressed structure, 
while the other part had a faint yellow tint belonging to a 
dilated structure. This appearance arose from the piece of 
glass CD not being placed in the middle of the tin cylinder as 
shown in the figure. The distance Ee was 0.74 of an inch, 
while Ff was 0.97, and as the dilating force was greater in 
the direction f F than in the opposite direction eE, and the 
resistances unequal, a slight concavity would take place at e, 
and produce the black space, and the two opposite structures. 
Scholium. 
The results contained in this Proposition lead to the con- 
struction of new instruments for measuring the contraction 
and dilatation of all substances whatever, whether they are 
produced by variations in their temperature, or in their humi- 
dity. Hence we obtain measures also of the degrees of tem- 
perature and humidity by which these mechanical changes 
are produced. 
A plate of glass inclosed in metal, as shown in Fig. 15, 
(PI. X.) forms a chromatic thermometer different from the one 
I have described in a former paper.* In the present instru- 
'V 
ment, the tints are produced by the difference of pressures 
upon the glass, occasioned by the difference of expansions 
arising from changes of temperature ; whereas, in the other 
instrument, the tints originate immediately from the changes 
of temperature. The exterior case of the thermometer repre- 
* Phil. Trans. 1,8 1.6, p, 108. 
