that planet, and all we know or suspect of the attendants on Sa- 

 turn is in obedience to the same law. Why this is so is unknown 

 — it is one of those dark points that stir us to profounder inquiry 

 concerning the scheme in which we are ; but it is not difficult to 

 trace some of its effects. Owing to this arrangement the moon 

 always turns the same face toivards the earth. As she goes her 

 monthly rounds, she revolves proportionally on her axis — each 

 spot keeping its relative position towards us ; and if it were not 

 that her velocity in her orbit is slightly irregular, we should 

 never see anything of her opposite hemisphere," p. 159. Other 

 astronomers state the doctrine in very similar terms, but perhaps 

 the best and most explicit expression of the received doctrine of 

 lunar rotation is that given by M. S. D. Poisson in his Traite 

 de Mecanique, 2de edition, Tome 11. p. 179-180, in these words: 

 " Le mouvemont de translation pent etre revolutive autour d'un 

 autre corps en repos, ou lui meme in mouvement. II n'y a pas 

 de rotation toutes les fois qu'une face ou une section determine 

 du mobile reste constamment parallele a elle meme ; il y a, au 

 contraire, rotation dans le meme temps que la revolution, lorsque 

 le mobile tourne constamment la meme face vers le corps cen- 

 tral. C'est ce second cas que a lieu dans le mouvement des sa- 

 tellites autour de leurs planetes. La lune tourne toujours la 

 meme face vers la terre, et le rayon vecteur qui va du centre de 

 la terre au centre de la lune rencontre toujours en un meme 

 point la surface du satellite, d'ou il resulte que la rotation de la 

 lune sur elle meme, et sa revolution autour de la terre s'achevent 

 dans un meme temps." '' The movement of translation (of a 

 progressing body) may be revolutive round another body at rest, 

 or itself in motion. There is no rotation when one face or a de- 

 terminate section of the moving body remains constantly parallel 

 to itself ; there is^ on the contrary, rotation in the same time as 

 the revolution when the moving body turns constantly the same 

 face towards the central body. This second case is that of the 

 satellites in moving around their primaries. The moon turns 

 always the same face to the earth, and the radius-vector, which 

 connects the earth's centre with the moon's, cuts the surface of 

 the satellite always at the same point ; whence it results that the 

 moon's rotation on her own axis, and her revolution about the 

 earth are accomplished in the same time." 



The question at issue is simply this : astronomers say that it is 

 possible for the moon, or any other body revolving in an orbit, 

 to travel round that orbit, and present every part of its circum- 

 ference in succession to the orbit's centre, without turning 

 round on its own axis; that the moon, revolving freely in space, 

 should exhibit this phenomenon ; but as she does not, and, on 

 the contrary, presents to the earth the same face continually, it 



