466 



NA TURE 



[Sept. ii, 18S4 



reflection of the rays by the diagonal mirrors, I have con- 

 structed a special apparatus, which by means of a 

 parallactic mechanism faces the sun at right angles during 

 observations. It consists principally of two s nail mirrors, 

 manufactured of the same materials as the reflector, placed 

 diagonally at right angles to each other; a thermometer 

 being applied between the two, whose stem points towards 

 the sun. The direct solar rays entering through perfora- 

 tions of an appropriate shade, and reflected by the in- 

 clined minors, act simultaneously on opposite sides of the 



bulb. The mean result of repeated trials, all differing 

 but slightly, show that the energy of the direct solar rays 

 acting on the polygonal reflector is reduced C235 before 

 reaching the heater. 



In accordance with the previous article, the investiga- 

 tion has been based on the assumption that, the tempera- 

 tures produced by radiant heat at given distances from 

 its source are inversely as the diffusion of the rays at those 

 distances. In other words, the temperature produced by 

 so/a?' radiation is as the density of the rays. 



Captain Ericsson's Solar Pyrometer, erected at New York, 1884. 



It will be remembered that Sir Isaac Newton, in esti- 

 mating the temperature to which the comet of 16S0 was 

 subjected when nearest to the sun, based his calculations 

 on the result of his practical observations that the maxi- 

 mum temperature produced by solar radiation was one- 

 third of that of boiling water. Modern resean I 

 that the observer of 1680 underrated solar intensity onl; 



5° for the latitude of London. The distance of the let 



from the centre of the sun being to the distance of the 



earth from the same as 6 to 1000, the author of the 

 " Principia " asserted that the density of the rays was as 

 1000' to 6 2 = 28,000 to I ; hence the comet was subjected 



180° 

 to a temperature of 28,000 X ~~ r" = 1,680,000", an 



intensity exactly " 2000 times greater than that of red- 

 hot iron " at a temperature of 840 . The distance of the 

 comet from the solar surface being equal to one-third of 

 the sun's radius, it will be seen that, in accordance with 



