PHYSICS OF THE NINETEENTH CENT HEAT. 507 



experiments with this apparatus gave the velocity of light as 314,262,944 

 metres, or 195,344 miles, per second. 



The distance between the stations in M. Fizeau's experiments was 

 more than five miles, and when we consider the enormous velocity of 

 light, and that it accomplished the double journey in about the 18 i 44 th 

 part of a second, we cannot but admire the ingenuity of the method. 

 This was the first time in which the speed of light had been directly 

 measured. But soon afterwards Fizeau's compatriot, Foucault, actually 

 measured the time taken by a ray of light in traversing the space of 

 26 feet. The principle Foucault employed was the same as that 

 adopted by Wheatstone in his measurement of the duration of the 

 electric spark. The displacement of the reflected image of a fine wire, 

 by the angular change in the position of a small mirror making from 

 400 to 500 revolutions per second, gave the means of estimating the 

 time required by the light to traverse a certain distance. A descrip- 

 tion of the apparatus would introduce more technical details than 

 would be appropriate in this place. It may be interesting to remark, 

 however, that the number of revolutions per second made by the re- 

 volving mirror was known by the pitch of the 

 sound it emitted. 



Some of the phenomena of radiant heat were 

 to a certain extent investigated by Sir W. Her- 

 schel. It was he who discovered that the visible 

 spectrum (page 217) of a solar beam does not 

 represent the whole of the rays that are refracted 

 by the prism ; that the spectrum is extended 

 beyond the red end in rays of obscure heat. 

 Sir John Leslie in 1811 examined the radiating 

 power of different substances. He devised the 

 instrument called the Differential Air Thermo- 

 meter (Fig. 231), which consisted of a narrow 

 upright U-shaped glass tube having each branch 

 terminated by a glass bulb. The tube, hermeti- 

 cally closed, contained air, and a little liquid in 

 the bend of the tube. Whenever the tempera- 

 ture of one of these bulbs exceeded that of the 



other, the expansion of the air in the hotter bulb moved the column 

 of liquid toward the colder bulb. Leslie used cubical vessels filled 

 with hot water and coated on the four vertical sides with the various 

 substances of which he desired to ascertain the radiating power for low 

 heat. The face of the cube to be tested was placed before a concave 

 mirror, from which the rays of heat were reflected upon the bulb of 

 the air thermometer. He found that a surface covered with lamp- 

 black emitted at the same temperature more heat than a surface of 

 polished nickel in the proportion of 100 to 12. Other substances were 

 intermediate. The radiation was found to depend upon the condition 



FIG. 231. 



