PYRONOMICS. 87 
the bar mn lie opposite to two lateral apertures, closed by glass plates. 
For higher temperatures, the bar is placed on a support, likewise iron, in a 
brick furnace, in which are small holes opposite to the telescopes. 
As it is in our power, from the known temperature, to determine the 
extension of any body, so, conversely, from the known extension of a body, 
the temperature to which it is exposed may be ascertained. The ordinary 
thermometers range only to about 360°C., or 660°F., above which mercury is 
‘converted into vapor, so that it is the melting points of such bodies only as 
are below this degree, such as tin, tellurium, bismuth, and lead, that can be 
ascertained by the mercurial thermometer. All other metals have higher 
melting points, and from the expansion of these it has been attempted to 
ascertain elevated degrees of temperature. Muschenbroek, in 1769, invented 
the metal pyrometer, which, in its general features, agrees with the apparatns 
described above for measuring the extension of a metallic bar. The pyro- 
meter invented by Wedgewood in 1782, depends upon a different principle, 
namely, that of the contraction of a certain kind of clay by heat. Small 
cylinders of this clay were carefully measured before and after the exposure 
to heat, and from the difference of length the intensity of heat was deter- 
mined. The great defect here, however, was, that even in the most carefully 
constructed cylinders, the contraction was not sufficiently uniform. 
Daniell’s pyrometer possesses fewer defects than any yet constructed. 
The indications of this instrument rest upon the difference of expansion of 
an iron or platinum rod, in a tube of plumbago, when extended by a great 
heat. A metal bar, shorter than the tube, is placed in it, and over the bar is 
placed a shorter bar of clay, which, placed in the opening of the tube, serves 
as an index by being placed upon the bar in the tube, and attached in such 
a manner, by means of a small plate of platinum, as to move only with a 
certain degree of friction. 
If, now, the point be marked where, in an unheated state, the clay bar 
meets the tube, and the apparatus be then exposed to heat, the expansion of 
the metal will drive out the clay to a certain point, at which, owing to 
the friction, it will remain on cooling. The amount by which the clay has 
been protruded will give the elongation of the platinum bar. The disad- 
vantage, in this case, is, that the extension of the plumbago tube itself cannot 
be determined with suflicient accuracy. 
From the measured linear expansion of bodies, their cubic expansion may 
readily be ascertained, it being necessary only to find the coefficients for the 
first. The coefficient of expansion for solid bodies will be three times as 
great as that for these linear expansions, as these bodies are extended in 
height and breadth, as well as in length. 
The expansion of solids by heat, and their contraction by cold, are 
powerful forces; for if a weight of 1000lbs. be necessary to compress 
a body as much as it is contracted by a diminution in temperature of one 
degree, then this diminution will push or pull an obstacle with a force of 
1000lbs. Use has been made of this force to restore walls, by means of the 
contraction of iron braces, to a perpendicular from which they had swerved, 
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