260 
SECOND REPORT— 1832 . 
Thermometer cooled from 212° to 32° Fahrenheit: 
In Vacuo ....... in 10 m 5 sec 
Air ........ . 73 
Water. 15 
Mercury ....... 0 36 
Dulong and Petit, in their elaborate researches on the cooling 
of bodies, have investigated the law of cooling in the most perfect 
vacuum they could form : but they admit that there was always 
a minute portion of air present. The radiation therefore of 
heat in an absolute vacuum is by no means conclusively esta¬ 
blished. (See Annals of Phil. vol. xiii. p. 241.) 
3. ) Professor Leslie ascertained, That the effect from a mass 
of given size is nearly proportional to the angle which it sub¬ 
tends at the thermometer; and that the heat suffers little or no 
diminution in its passage through the air. 
The radiation is most copious in the direction perpendicular 
to a plane surface of the hot mass, and is proportional to 
the sine of its inclination to the direction of the thermometer* 
(Inquiry into the Nature and Propagation of Heat , p. 51, &c.) 
For the same position the effect is proportional to the excess 
of temperature of the hot body above that of the air. 
4. ) Pictet made an attempt to estimate the velocity with which 
heat radiates, by means of concave reflectors at 69 feet distance. 
The effect on the focal thermometer was absolutely instanta¬ 
neous. (Essais de Phys.) 
b.) Reflexion of simple heat from nonluminous hot bodies . 
1. ) The general principles are established by Professor Les¬ 
lie. ( Inquiry , pp. 14, 51.) 
2. ) He shows that the quantity of heat reflected is propor¬ 
tional to the sine of incidence on a plane surface. 
3. ) It is affected by the polish of the surface. (Leslie, In¬ 
quiry, pp. 81, 20, 98, 106.) 
4. ) The most exact experiments are those made with con¬ 
jugate concave reflectors ; a ball of iron below luminosity in one 
focus, a thermometer in the other: a glass of boiling water 
may be substituted for the iron ball. In either case a great 
effect is produced in the opposite focus, though little out of 
it. (Saussure, Voyages, t. iv. p. 120; Sir W. Herschel, Phil, 
Trans . 1808, p. 305.) ' 
Professor Leslie made extensive use of reflectors, but ob- 
