refraction to glass, &c, by pressure . 17 1 
sented by AB in Fig. 17, (PI. X.) may even be made of iron, 
brass, or any other metal that is not easily fused. And when 
this ring is brought to a high degree of heat, fluid lead, or tin, 
may be poured into the centre of it, so as to be immediately 
in contact with the piece of glass CD. 
A chromatic hygrometer may be constructed by surrounding 
a piece of glass with a mass of any hygrometric substance, that 
readily absorbs moisture. This substance may be advantage- 
ously inclosed in a piece of glass or earthen ware, perforated 
in different places to admit the air freely. 
Instead of measuring the direct pressure occasioned by con- 
traction or expansion, the magnitude of the scale would be in- 
creased by employing these forces to bend a long slip of glass, 
as in Fig. 18, (PL X.) where AB is the glass resting against 
fixed supports A,B, and CD a mass of lead, or a hygrometric 
substance, resisted by the support E F, and altering the cur- 
vature of AB, by its contractions or dilatations. If the expand- 
ing mass CD Fig. 19, (PL X. ) is made to act on the two 
extremities, A,B of the glass plate fixed at the middle M, it 
may sometimes be concave towards C, and sometimes con- 
vex, and the limit between these two states may be taken for 
the zero of the scale. 
