under the protecting wing of their immediate superior, the sweep of their other 

 sun in its perihelion passage round their own, might carry them off, or whirl 

 them into orbits utterly incompatible with the conditions necessary for the ex- 

 istence of their inhabitants. It must be acknowledged that such a spectacle 

 presents a strangely wild and novel field for speculative excursions, and one 

 which it is difficult to avoid luxuriating in." 



Those who are unaccustomed to the consideration of geometrical questions 

 will be enabled to acquire a tolerably clear idea of such a system as has been 

 just described by means of the annexed diagram. 



Fig. 21. 



s. 



\ 



I 



\ 



/ 



>* 



The larger sun with its planets is represented at S, in the focus of an ellipse, 

 in which the lesser sun accompanied by its planets moves. At A, this latter 

 sun is in its perihelion, and nearest to the greater sun S. Moving in its peri- 

 odical course to B, it is at its mean distance from the sun S. At C it is at 

 aphelion, or its most distant point, and finally returns through D to its perihelion 

 A. The sun S, because of its vast distance from the system A, would appear 

 to the inhabitants of the planets of the system A much smaller than their proper 

 sun, but on the other hand this effect of distance would be to a certain extent 

 compensated by its greatly superior magnitude ; for analogy justifies the infer- 

 ence that the sun S is greater than the sun A in a proportion equal to that of 

 the magnitude of our sun to one of the planets. The inhabitants of the planets 

 of the system A will then behold the spectacle of two suns in their firmament. 

 The annual motion of one of these suns will be determined by the motion of ! 

 the planet itself in its orbit, but that of the other and more distant sun will be 

 determined by the period of the lesser sun around the greater in the orbit 

 A B C D. The rotation of the planets on their axes will produce two days 

 of equal length, but not commencing or ending simultaneously. There will be 

 in general two sunrises and two sunsets ! When a planet is situate in the part 

 of its orbit between the two suns, there will be no night. The two suns will 

 then be placed exactly as our sun and moon are placed when the moon is full. 

 When the one sun sets the other will rise, and when the one rises the other 

 will set. There will be, therefore, continual day. On the other hand, when 



VOL* II.— 34 



