307 



Tohn Herschel 397 



(1833) of about 2500, of which some 500 were new and 

 2000 were his father's, a few being due to other observers ; 

 incidentally more than 3000 pairs of stars close enough 

 together to be worth recording as double stars were observed. 



307. Then followed his well-known expedition to the 

 Cape of Good Hope (1833-1838), where he "swept" the 

 southern skies in very much the same way in which his 

 father had explored the regions visible in our latitude. 

 Some 1200 double and multiple stars, and a rather larger 

 number of new nebulae, were discovered and studied, while 

 about 500 known nebulae were re-observed ; star-gauging on 

 William Herschel's lines was also carried out on an extensive 

 scale. A number of special observations of interest were 

 made almost incidentally during this survey : the remarkable 

 variable star v\ Argus and the nebula surrounding it (a 

 modern photograph of which is reproduced in fig. 100), the 

 wonderful collections of nebulae clusters and stars, known 

 as the Nubeculae or Magellanic Clouds, and Halley's comet 

 were studied in turn ; and the two faintest satellites of 

 Saturn then known (chapter xn., 255) were seen again 

 for the first time since the death of their discoverer. 



An important investigation of a somewhat different 

 character that of the amount of heat received from the 

 sun was also carried out (1837) during Herschel's residence 

 at the Cape ; and the result agreed satisfactorily with that 

 of an independent inquiry made at the same time in France 

 by Claude Servais Mathias Pouillet (1791-1868). In both 

 cases the heat received on a given area of the earth in a 

 given time from direct sunshine was measured ; and allow- 

 ance being made for the heat stopped in the atmosphere 

 as the sun's rays passed through it, an estimate was formed 

 of the total amount of heat received annually by the earth 

 from the sun, and hence of the total amount radiated by 

 the sun in all directions, an insignificant fraction of which 

 (one part in 2,000,000,000) is alone intercepted by the 

 earth. But the allowance for the heat intercepted in our 

 atmosphere was necessarily uncertain, and later work, in 

 particular that of Dr. S. P. Langley in 1 880-81, shews that 

 it was very much under-estimated by both Herschel and 

 Pouillet. According to Herschel's results, the heat received 

 annually from the sun including that intercepted in the 



