HE A T APPARA TUS. 1 43 



* f Flux of Heat " across a plane unit of area, at any point of an 

 isotropic solid, is thus proportional to the conducting power and 

 to the gradient of descent of temperature per unit of length in the 

 direction perpendicular to the plane of the area. 



Apparatus for measuring Conductivity. [Peclet, Biot, Langberg, 

 Forbes, Angstrom, &c.] 



Apparatus for showing difference of Conducting power in dif- 

 ferent directions. [De Senarmont, &c.] 



Propagation of surface heat downwards into the crust of the 

 earth. [Quetelet, Leslie, Forbes, &c.] 



Loss of heat by the eajth in consequence of the observed 

 .gradual increase of temperature with depth under the surface. 



Instruments for measuring temperature in borings, &c. 



Applications of imperfect conductors to prevent waste of heat, 

 as Jackets for the Cylinders of Steam-Engines, &c., apparatus for 

 keeping Ice, Solid Carbonic Acid, &c. 



(b) RADIATION. (See also under Light, p. 121.) 



Apparatus for the measurement of Radiant Heat. 



Leslie's Differential Thermometer, and its varieties. 



Melloni and Forbes, Thermo-electric Pile and Galvanometer. 



Thermo-electric arrangements for heat of Moon and Stars, and 

 for comparative temperature of sun-spots and faculae. [Airy, Lord 

 Rosse, Secchi, &c.] 



Apparatus for absolute measure of Radiation. 



Apparatus for rate of cooling in vacuo. 



Pyrheliometer. 



Radiometer. 



Apparatus for the full comparison of the behaviour of Light with 

 that of Radiant Heat, as to 



Interference, 

 Diffraction, 

 Absorption, 

 Reflexion, 



