PYRONOMICS. 105 
2. The differential thermometer ot Leslie (see accompanying 
figure) consists of a curved tube with bulbs blown at both ends, 
and standing on a foot. The tube is filled with a colored 
liquid. If the one bulb be placed in the focus of a concave 
mirror and the other out of it, at the least heating of the first 
bulb, the liquid in the tube will change its position; and the 
amount of this change may be read off on the scale. 
3. Melloni’s thermo-multiplier ( pl. 19, figs. 37 and 37%). This 
consists of a sensitive multiplier and a thermo-electric pile. com- 
posed of twenty-five to thirty fine needles or bars of antimony 
and bismuth, connected alternately at their extremities, and sepa- 
rated laterally by some non-conductor, the whole united into a 
compact bundle. Each of the terminating elements of the pile is 
connected with one of the projecting pins, which thus form the 
poles of the pile. The pile is lamp-blacked at both ends, and, 
with its covering, placed on a foot atp. The bonnets a and ), of 
which bd is conical, serve to keep off from the pile all lateral radi- 
ations. In addition to this, b serves to concentrate the rays of heat coming from 
that side. The copper wire, twenty-two to twenty-four feet long, forming 
the galvanometer, is wound upon a metal frame. The carefully compensated 
magnetic needles are, as shown in fig. 37’, united together and suspended by 
a fibre of raw silk, hanging in the middle of a glass bell. By turning the 
button f, the fibre with the needles may be slightly elevated or depressed. 
The extensible spiral wires, g, serve to connect the poles of the pile or 
battery with the extremities of the multiplier wire. The entire apparatus 
is so placed and adjusted upon a sufficiently firm table as that the thread 
shall hang in the middle of the graduated circle, and the needles point to 
the zero of the graduation. The least change of temperature between the 
two extremities of the pile produces an immediate deviation of the needle, 
which may be measured on the graduated circle. 
If, in the focus of a mirror, any one of the above-mentioned pieces of 
apparatus be introduced, and in the other a body whose surface amounts te 
one third to three fourths of an inch, the apparatus will show that this body 
constantly radiates heat. even if its temperature be but little higher than that 
of the surrounding bodies. Thus, in a cold room, melting ice will radiate 
heat, and thereby elevate the temperature in the other focus. If the tempe- 
rature of the room be above 32°, the thermometer in the focus of one mirror 
will sink if ice be placed in that of the other. This, however, is not an 
instance of radiation of cold, but simply an inversion of the usual operation ; 
the thermometer is now the radiating body. giving off its heat to the ice. 
If Melloni’s thermo-multiplier be employed, a mirror is not necessary, for by 
attaching the conical hood, b, the rays are concentrated by it so strongly, 
that even if the hand be held against the opening of the hood, at a distance 
of several steps, the radiation from the former will be sufficient to produce a 
very sensible deflection of the needle. 
Heat rays, on striking any body, are either absorbed, reflected, or trans- 
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