THE SUN'S HEAT. 21 



loss of heat by transmission through the glass, the temperature at the 

 focus should be the same as that of a point placed at such a distance 

 from the sun that the solar disk would seem just as large as the lens 

 itself viewed from its own focus. 



The most powerful lens yet constructed thus virtually transports 

 an object at its focus to within about 250,000 miles of the sun's sur- 

 face, and in this focus the most refractory substances platinum, fire- 

 clay, the diamond itself are either instantly melted or dissipated in 

 vapor. There can be no doubt that, if the sun were to come as near us 

 as the moon, the solid earth would melt like wax. 



In 1878 Professor Langley made a careful comparison between the 

 radiation of the sun and that of the molten metal in a Bessemer "con- 

 verter " when at its greatest heat. By a very ingenious arrangement 

 he brouffht the solar heat and that from the metal to confront each 

 other upon the faces of a thermopile ; and he found that, even neglect- 

 ing all corrections for the loss of solar heat by transmission through 

 the smoky atmosphere of Pittsburgh, and by the reflections which 

 brought it to his apparatus, the sun's radiation was eighty-seven times 

 as powerful as that from the converter, surface for surface. Had the 

 just corrections been ascertained and applied (a matter, however, of 

 extreme difficulty, and even impossible under the circumstances), the 

 ratio would be increased from eighty-seven to more than one hundred 

 certainly, and pei'haps to one hundred and fifty. 



As to the temperature of the metal in the converter. Professor 

 Langley considers that it must have been above that of the fusion of 

 platinum, because platinum wire held over the mouth of the converter 

 just before pouring, or in the stream of metal, melts immediately. 

 There may be some question, however, whether the melting of the 

 wire really indicates quite so high a temperature, since fluid iron and 

 its vapor attack platinum in something the same way as mercury and 

 its vapor attack gold and silver. Similar conclusions as to the inten- 

 sity of the solar temperature follow from investigations by Soret and 

 others, as to the penetrating power of the sun's rays ; and from a 

 comparison with artificial sources of heat in respect to the relative 

 proportion of the rays of different wave-lengths in the total radia- 

 tion. A body of low temperature emits an enormous proportion of 

 slowly vibrating, invisible vibrations, while, as the temperature rises, 

 the shorter waves become proportionally more and more abundant. 

 Thus, in the composition of a body's radiation, we get some clew to 

 its temperature. Hitherto all such tests concur in putting the sun's 

 temperature high above that of any known terrestrial flame. 



And now we come to questions like these : How is such a heat 

 maintained ? How long has it lasted already how long will it con- 

 tinue are there any signs of either increase or diminution ? ques- 

 tions to which, in the present state of science, only somewhat vague 

 and unsatisfactory replies are possible. 



