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remain in the same invariable relation to the radius-vector and 

 the centre of the orbit. But, at the first quarter, or at any other 

 intermediate point short of the half- circumference, set the needle 

 at liberty and it will fly back to its natural position, directly north 

 and south, while the box and the radius will remain as before, in 

 the same undisturbed relation to the orbit's centre. The cause of 

 the needle's rotation is well known, and sufficient to account for 

 the phenomenon ; while there is no apparent or probable cause of 

 rotation in either the box or the radius. It is, therefore, the 

 needle that rotates, and neither the box nor the radius ; yet the 

 effect of that rotation is to keep the needle in the same direction 

 oi parallelism^ the very result which, in the moon's case, astrono- 

 mers would ascribe to her wo?z-rotation. 



A ship sailing round the world is placed in circumstances pre- 

 cisely similar to those of the moon. It describes a circular path- 

 way on the earth's surface, with just as much freedom of move- 

 ment -as the moon herself possesses in her orbit. If it were to 

 move under the influence of the centrifugal or tangential force 

 alone, represented in its case by the wind or steam that drives it 

 along, it would move straight forward, and continually lift every 

 point of its circumference in succession from the water, just as 

 the moon, if equally free, as astronomers assume that she is, 

 would turn her points successively away from the earth. But 

 the moon, it is said, possesses a power of rotating on her axis in 

 a contrary direction, and that power " brings''^ the outward turn- 

 ing point continually " bacK'' to the radius. It is not easy, in- 

 deed, to conceive what can be meant by '' bringing back" what 

 not only never goes away, but on the contrary keeps invariably 

 the same position. To speak, therefore, more correctly, we 

 should say that the alleged power, if it exist, keeps i\iQ same point 

 of the moon's circumference continually in one with the radius- 

 vector, and prevents its ever moving away at all, so as to require 

 *"• bringing bacV^ It just neutralizes the natural tendency to turn 

 her circumference outwards, which the moon would have if she were 

 free. But the ship, it will be admitted, does not possess such a 

 power to bring its prow continually back to the water, and so to 

 keep its keel continually directed to the centre of the earth. By 

 what means then does it exhibit, in precisely similar circum- 

 stances, the same phenomenon as the moon, if it want what is 

 considered indispensable to the production of that phenomenon in 

 the lunar globe, that ship sailing m the sky ? Why does not 

 the ship present its topmast and keel alternately, and indeed 

 every point of its perpendicular circumference in succession, to 

 the earth's centre, which is the centre of the orbit in which it 

 revolves? It must either do this or bring the same point con- 

 tinually back to its radius-vector by virtue of some power in- 



