Jan. 20, 1876] 



NATURE 



225 



from the centre of the disc, will appear to be reduced to the 

 space a cos Q ; the intensity of its light must therefore be in- 

 creased in the ratio of unity to cos 0." 



2. In order to disprove the correctness of the stated demon- 

 stration, I have measured the relative thermal energy of rays 

 projected m different directions from an incandescent metallic 

 disc, by the following method > — Fig. 2 represents section of a 

 conical vessel covered by a movable semi-spherical top, the vessel 

 being surrounded by a jacket through which water may be cir- 

 culated. A revolving circular disc, a a, composed of cast iron, 

 the back being semi-spherical and protected by fire-clay, is sus- 

 pended across the top of the conical vessel supported by 

 horizontal journals attached at opposite sides. The angular 

 position of the disc is r^julated by a radial handle, b, connected 

 to one of the journals ; the exact inclmation to the vertical line 

 being ascertained by means of a graduated quadrant, d. An 

 instrument, c, capable of indicating the intensity of the radiant 

 heat transmitted by the incandescent disc, is applied at the 

 bottom of the conical vessel. The mode of conducting the 

 experiment is extremely simple. The movable cover and its 

 lining of fire-clay having been removed, the cast-iron disc is 

 heated in an air-furnace to a temperature of 1,800' F. It is then 

 removed by appropriate longs, and susp)ended over the conical 

 vessel, the lining and cover being quickly replaced. The tem- 

 perature, shown by the instrument at the bottom of the conical 

 vessel, resulting from the action of the radiant heat of the disc, 

 is then recorded for every tenth d^ree of inclination. The inves- 

 tigation, it may be briefly stated, shows that the temperatures 

 imparted by radiation to the recording instrument is e.xactly as 

 the sines of the angles of inclination of the disc. Hence, at an in- 

 clination of 10° to the vertical hne, the temperature imparted to the 

 thermometer is scaVcelv one-sixth of thit imoarted when the disc 



faces the thermometer at right angles ; yet in both cases an ejmil 

 amount of surface of an eqttal degree of incandeicence is radiating 

 towarf s the instrument ! Laplace and his followers have evi- 

 dently overlooked this important and somewhat anomalous fact, 

 proving that radiation emanating from heated bodies is incapable 

 of exerting full enegy in more than one direction. Our practical 

 experiments with the revolving incandescent disc have thus fully 

 demonstrated the truth of the proposition intended to be esta- 

 blished, namely, that the rays emanating from incandescent 

 planes do not transmit heat of equal energy in all directions, the 

 energy transmitted being as stated, proportionate to the sines of 

 their angle of inclination to the radiating surface. 



3. The next step in the investigation of solar heat, before 

 adverted to, was that of measuring the radiant energy trans- 

 mitted in a given direction by an incandescent solid metallic i 

 sphere. For this purpose I employed a double conical vessel 

 similar to the one represented in Fig. 2, the incandescent sphere 

 being suspended over the conical vessel in the same manner as 

 the revolving disc The nature of the arrangement will be 

 readily understood by inspecting the annexed diagram, which 

 represents four spheres. Figs. 3, 4, 5, and 6, each sphere being 

 divided into four zones, \, B, c, and D, occuppng unequal arcs, 

 but containing equal convex areas. Semi-spherical screens com- 

 posed of non-conducting substances were applied below each 

 sphere, provided with annular openings, arranged as shown in 

 the diagram. Through these annular openings the radiant heat 

 from the incandescent zones, D, c, B, and .K, was transmitted 

 to the thermometers, f g, h, and k, respectively. Pere Secchi, 

 and other followers of Laplace, will be surprised to learn that 

 when the suspended sphere was maintained at a temperature of 

 1,800^ F., the radiation from the zone c, Fig. 4, imparted a 



temperature of 27° "49 F. to the thermometer g, while the radia- 

 tion from the zone a. Fig. 6, imparted only 6° '19 F. to the ther- 

 mometer k. Let us bear in mind that the radiating surface / m 

 of the zone K is equal to the radiating surface p ^ of the zone c. 

 The stated great difference of temperature produced by the radi- 

 ation from zones of equal area furnishes additional proof that 

 Laplace based his remarkable analysis on false premises. " The 

 sun's disc ought to appear more brilliant towards the border, 

 because viewed under a less angle," we are told by the great 

 analyst. The instituted practical tests, however, prove posi- 

 tively that the energy of the xz-j^ projected from the border of 

 an incandescent sphere is greatly diminished because vieuvd 

 under a less angle from the point occupied by the recording 

 thermometer. 



nc. 2. 



4. The result of our experiment with the revolving'^incan- 

 descent disc shows that if the small arc b a, in Fig. i, be reduced 

 until the field represented by 1/ a' becomes equal to the field 

 represented by (' d', the radiant energy transmitted through each 

 of those fields will be alike ; the reason being that the number 

 of rays of diminished intenjity passing through d d' will be as 

 much greater than the number of rajrs of maximum intensity 

 passing through 1/ a', ascd is greater than the reduced ba = fc. 

 It should be observed that r ^ is so small that we may without 

 appreciable error regard it as a straight base, and fc as the sine 

 of the angle cdf. It follows from this demonstration that if the 

 solar atmosphere exerted no retarding influence, the radiant 

 heat transmitted towards the earth would be alike for equal areas 

 of the solar disc — more correctly, for areas subtending equal 

 angles, since the receding part of the solar surface is at a greater 

 distance from the earth than the central part 



Encouraged by the practical result of the instituted investiga- 



