A. MATHEMATICS AND ASTRONOMY. 43 



a thick liquid layer over the exterior of the solid nucleus. If, 

 now, this exterior layer had, on the average, a density of 1.54 

 times the density of the nucleus, we should be able to ex- 

 plain the large flattening observed at present on the planet 

 Mars. Comptes Rendus, torn, lxxviii., p. 1557. 



NATURE OF THE SUN'S SPOTS. 



To the question of the nature of the solar spots, Professor 

 Sporer has contributed somewhat, by applying to the ob- 

 servations quoted by Faye in support of his theory the very 

 method of investigation suggested by the latter. While 

 Faye defends the idea that the spots are deeper than the 

 solar surface, Professor Sporer shows very satisfactorily that 

 they are, on the contrary, above the surface, and must be 

 considered as cloud-like objects. Ileis* Wochenschrift, 1874, 

 p. 160. 



herschel's observations of the SATELLITES OF URANUS. 



As is well known, Sir William Herschel in the early part 

 of this century concluded, from the numerous observations 

 that he had made upon the planet Uranus, that it was at- 

 tended by six satellites. Since his day no one, not even his 

 son, Sir John Herschel, using the same telescope, has been 

 able to perceive more than two of these six satellites, until it 

 was reserved for Lassell, in 1845, to discover two others 

 which were much nearer to the planet. The two interior 

 satellites discovered by Lassell were, however, evidently 

 entirely different from the two inner satellites claimed by 

 Herschel. The study of Uranus that has recently been made 

 at Washington, by means of the twenty - six -inch Clark 

 refractor, has also failed to show the existence of any of 

 the four missing Iierschelian satellites; while the four ob- 

 served by Lassell have been very frequently seen, and their 

 position accurately observed. Professor Holden, of the ob- 

 servatory, has undertaken to discuss the interesting question 

 as to whether the elder Herschel did not actually observe 

 the two inner satellites of Lassell ; namely, Ariel and Urn- 

 briel. By careful computations backward, based upon the 

 result of Newcomb's observations, Professor Holden is led to 

 the conclusion that it is certain that Herschel saw Ariel in 

 1798, and that he probably saw it in 1790. It is also certain 



