12 



OUR PRESENT KNOWLEDGE OF THE 

 PLANETARY WORLD. 



By the Rev. S. J. PERRY, F.R.S., F.R.A.S. Januanj \Wi, 188G. 



The lecture opened by a few words on the Sun as Piuler of 

 the Planetary World, the lecturer showing, by aid of plioto- 

 graphs projected on the screen, the actual state of the solar 

 surface, and of the atmospheres surrounding it. The size of the 

 Sun was then compared with that of the planets, before passing 

 on to a brief consideration of Kepler's empirical laws of planetary 

 motion, and of their later confirmation by Newton. The elliptic 

 motion and the varying velocity of a planet in its orbit were 

 deduced from the law of universal gravitation ; and then the 

 relative distances of the chief planets from the Sun were shewn 

 in different waj^s, in order to gain the clearest notion of the ex- 

 tent of our solar system. 



The separate planets were next taken in their order, starting 

 from the sun. The iutermercuvial planet, whose orbit was cal- 

 culated by Le Verrier from the perturbations of the node of 

 mercury, and which was possibly observed by Prof. Watson and 

 Mr. Swift during the total eclipse of 1878, was lightly touched 

 upon, as Vulcan's existence still requires confirmation. The 

 drawings of Mercury and Venus by Schroeter, and the spots 

 observed by Bianchini, with the phases as illustrated by De- 

 launay and Newcomb, completed the review of the inferior 

 planets ; the hypothetical planet Neith being disposed of by the 

 theory of P. Thirion, who fully accounts for its appearances by 

 ice-crystals in the upper atmosphere. 



The Earth with its seasons, and some of Nasmyth's magnifi- 

 cent lunar sketches, led the way to the study of the superior 

 planets. 



The advantages of certain oppositions of Mars were made 

 evident from the jiosition of the orbits of the Earth and of Mars, 

 and the drawings by Secchi, Green, and Kuobel were exhibited 

 on the screen. These different series shew first the two poles 

 together in the same picture, and then North and South poles 

 separately, giving an excellent notion of the snow-clad portions ; 

 finally, the two hemispheres, as drawn by Newcomb, sum up 

 our knowledge of the outlines of seas and continents. The dis- 

 covery of the two small satellites of Mars by Asaph Hall was 



