206 PROGRESS OF SCIENCE IN THE CENTURY. 



other metals, besides a few non-metallic elements. 

 The clouds of the photosphere may be due to fog-pre- 

 cipitates from the cooling atmosphere, while depres- 

 sions or gaps in the photosphere probably give rise to 

 the phenomena of sun-spots. Herschel's idea of a 

 solid core cool and even habitable gave place to the 

 idea of an ocean of molten matter, but this, with 

 fuller knowledge of the conditions of the various 

 states of matter, has given place to the generally ac- 

 cepted view that the sun is in the main or wholly 

 gaseous. 



The Sun's Heat. About 1836, Sir John Herschel 

 at the Cape and Pouillet in France took a step which 

 meant much to the progress of physical astronomy. 

 It is hardly necessary to say that the step was one 

 of measurement. They tried to measure how much 

 of the sun's radiant energy is intercepted by the 

 earth a mere speck in the heavens (one part in two 

 thousand millions!) Although their estimates were 

 afterwards shown, by the work of Young, of Lang- 

 ley (1880-81), of Janssen (1897), and others to be 

 far under the mark, they were sufficient to indicate 

 the magnitude of the flood of energy which pours 

 forth from the hearth of our system. 



Herschel calculated that the heat received by the 

 earth in a year (including that caught in .the atmos- 

 phere) would suffice to melt a covering of ice 120 feet 

 thick over the whole surface of our planet; Young's 

 estimate leads to the result that " each square metre 

 of the Sun's surface pours out enough heat to main- 

 tain about half a dozen mighty Atlantic steamers at 

 their utmost speed night and day, from year's end to 

 year's end ; " * Langley remarks that " though there 



*Sir Robert Ball, The Story of the, Sun, 1893, p. 263. 



