86 PHYSICS. 
boiling points, so that 0° of Reaumur or Celsius ( R. or C.) = 32 F. It is 
customary in graduating for the Fahrenheit scale, to call the melting point 
of ice 32°, and marking off about 70° below this point equal to 70° 
above it. 
The measurement of temperature by means of the thermometer is exceed- 
ingly simple, al] that is necessary being to bring the bulb in communication 
with the temperature to be measured, and marking the elevation of the 
mercury after it has become stationary. 
As before observed, solids expand much less than liquids and gases, and 
must therefore be employed when high degrees of temperature are to be 
determined. As this expansion is of very small amount, it becomes neces- 
sary to resort to some contrivance for rendering it sensible. Now, if a rod 
be placed in contact with the short arm of a lever, the other being much 
longer, and its point serving as an index to a circularly graduated scale. then 
a slight expansion of the rod acting on the short arm will cause a consider- 
able traverse of the other over its graduated scale. A better arrangement 
for this purpose is the apparatus of Lavoisier and Laplace, represented in 
pl. 19, fig. 8. A rod, a, of the material to be tested lies horizontally upon 
glass bars, one end resting against a vertical glass bar, 6, which is sus- 
pended to a horizontal iron cross-bar, whose extremities are cemented into 
two massive stone pillars. The other end of the rod a is in immediate con- 
tact with a similar glass bar, c, carried by a bar, d, movable about its axis. 
To the prolongation of this latter bar, d, a telescope is attached, directed 
towards a distant scale. If, by the expansion of the rod a, the lower end of 
c be ever so slightly moved, the telescope will be turned, and its sight line, 
directed to another part of the scale, will indicate the amount of rotation. 
A box filled with heated water or oil is placed between the four pillars, for 
the purpose of heating the body to be examined, when dipped into it. This 
apparatus answers only for indicating temperatures below the boiling point 
of oil, as about 300° R = 707° F. 
For higher temperatures, the apparatus represented in jigs. 9—11 is 
better adapted: f is a strong iron plate, upon which is fastened an alidade, 
ab, turning about the point a. This carries a telescope, g, while a second 
telescope is fastened to the iron plate itself atc and d. <A rod, mn, is now 
brought in front of the two telescopes, so that its extremities fall in the 
centre of the field as indicated by the cross hairs. If the rod be increased 
in length to m’n, the extremity z remaining fixed, the alidade must be turned 
until the extremity m’ again falls in the centre of the field of the telescope g. 
The amount of this rotation is measured on a circular scale attached to the 
plate f. Ifthe proportion between am and ab be known, then, from the are 
4 . . . . mm' am . . 
VV’ is obtained the desired extension, mm’, Se ie The adjusting 
a 
screw, 7, serves to shift the alidade by a very slight amount, for the purpose 
of adjusting the telescope g. For temperatures below 300°R., a copper box 
is used, placed upon a furnace and filled with oil. The bar to be examined is 
placed upon an iron support, which rests on the box. The extremities of 
260 
