radiation from such an enclosure confirms in a remarkable manner Stefan's law of radiation, viz., R = T 4 .* Since therefore the results of several independent investigations corroborate this law, I have felt justified in applying it to the results of my observations.
On consideration it seemed that the most convenient form of radiator would be a long tube closed at one end, and uniformly heated in a gas furnace. Accordingly a porcelain tube, 2 feet in length and 1 inch internal diameter, was fitted into a Fletcher gas-tube furnace. This was afterwards changed for an iron tube, which was employed in the observations on September 30th, given below.
A plug of asbestos was inserted in the tube at about 10 inches from the end farthest from the radio-micrometer, and resting against this plug was the end of a Callendar platinum-resistance thermometer. This was connected with one of Professor Callendar's electric recorders, so that during an experiment the temperature of the tube was registered continuously on the paper wrapped round the drum of the instrument. In front of the open end of the tube, and between it and the radio-micrometer, was placed a large brass water-screen, through which a copious supply of water passed. In front of the aperture (B) of the radio-micrometer this screen was provided with a rectangular aperture. One side of this aperture was formed by a slide moved by a micrometer screw reading to O'Ol mm. By this means the area of this aperture at any time could be measured with precision. Its fixed sides were 5 mm. apart, and as the movable side had a range of 5 mm., the maximum area of the aperture was 25 sq. mm. The distance (d) of this aperture from the surface of the thermo couple was 66'3 mm.
To make an observation the tube was heated to as high a temperature as the furnace was capable of, and when a steady temperature had been obtained, the amount of radiant heat coming from the interior of the hot tube and passing into the aperture (B) of the radio-micro- meter was adjusted by the micrometer screw until a balance was obtained with the radiation coming from the Sun through the aperture (A).
If p is the angular semi-diameter of the sun, its radiation outside of our atmosphere is K.TT sin 2 p, where K is a constant depending on the sun's temperature.
Again, if a be the height of the slit through which the radiation from the hot tube reaches the radio-micrometer, and (3 its width, the radiation may with sufficient accuracy be expressed by Kx(3/d' 2 . Assuming Stefan's law, the radiation of the sun outside our atmo- sphere is #% sin 2 p, 6 being the effective temperature of the sun.
Stefan, 'Sitzber. d. k. Akad. zu Wien,' vol. 79, 1879 (Abth. 2), p. 391 ;; Boltzmann, 'Wied. Ann.,' TO!. 22, 1884; M. Planck, 'Drude Ann.,' vol. 1, No. 1,. 1900 ; Pascben, ' Wied. Ann.,' vole. 58, 60, 189fi, 1897 ; Lummer and Pringsheim,. ' Wied. Ann.,' vol. 63, 395, 1897.
