8 Biographical Mernoir of' Sir William Herschel. 



turn is a necessary consequence of the general principle of gra- 

 vitation. He deduced from his analysis the same duration of 

 ten hours and a-half, which the English astronomer afterwards 

 found by direct observation. The history of science presents 

 nothing more worthy of the attention of philosophers than this 

 wonderful accordance of theoretical inductions with the improve- 

 ment of the arts. 



HerschePs observations are so numerous and so varied, that 

 we cannot here attempt any exposition of their subjects. Most 

 of them have been confirmed and reduced to perfect certainty. 

 The instruments which he used, and which possess so many re- 

 markable advantages, are, however, liable to difficulties which 

 limit their utility. His largest telescopes ought always to be 

 considered rather as instruments of discovery than as instru- 

 ments of precise measurement. In this respect they are among 

 the most perfect productions of human ingenuity. 



We shall now speak of HerschePs views and experiments re- 

 lative to the physical properties of the solar rays. From a long 

 series of observations, made with powerful telescopes, he con- 

 cluded that the light does not emanate from the body of the 

 sun, but from certain shining and phosphoric clouds, which are 

 produced and developed in its atmosphere. He thought that 

 this immense ocean of light is violently agitated in its whole 

 depth ; that, when it is broken up, we perceive either the soHd 

 mass which is not so luminous, or its volcanic cavities, and that 

 this is the origin of those black and variable spots which are seen 

 on the sun's disk. Their extent is often much greater than the 

 whole surface of the terrestrial globe ; they disappear when a 

 calm is re-established in the solar atmosphere. It is well known 

 that these spots, first observed by Galileo, led to the discovery 

 of the sun's motion around its axis, and shewed that this motion 

 is accomplished in twenty-five days and a-half. 



The new improvements in optics afford a very unexpected 

 means of determining, whether it be true, as Herschel imagined, 

 that the solar light does not issue from an incandescent solid or 

 fluid. In fact, when such a body, raised to a very high tempe- 

 rature, becomes luminous, the rays which it gives off in all di- 

 rections do not come from the outer surface only, but are al- 

 so emitted like the rays of heat by a multitude of material points 



