43 



revolves from west to east, turning round on her axis once^ that 

 is, at the rate of one degree of her circumference for one degree 

 of her orhit, and precisely the same results will follow, without 

 tlie slightest difference in the phenomena. As before, the star 

 will tall so far short as to make tujo transits during the revolu- 

 tion ; or one transit more than any number of complete rotations 

 the earth may make. And by their own admission of precisely 

 similar results in the two cases, astronomers are precluded from 

 objecting to this conclusion of the argument, which entirely con- 

 futes their doctrine of the sidereal day's being the true measure 

 of the earth's complete rotation. 



It is true, indeed, that the outermost point of the earth's 

 circumference describes a much larger orbit than the inner- 

 most point, and from this fact astronomers infer that the point 

 «, after really completing one rotation, will take four minutes 

 more to make up the difference. But, it has been shown al- 

 ready, that this difierence in the dimensions of the orbits has 

 no effect whatever upon the rotation of the moving body ; and 

 that it only causes the axis to move forward more rapidly, and 

 change its place in relation to other bodies, while, with respect 

 to itself, all the parts of the body's internal mass and external 

 circumference remain in precisely the same relative positions, 

 whether they rotate or not. That is to say, if the body do not 

 rotate, that relation of the parts to the axis remains absolutely 

 invariable, however much the axis itself may change its place in 

 the orbit, and its relation to other bodies; if, on the contrary, 

 the body rotate, the rotating parts change their places just pre- 

 cisely to the same amount in the same time, as they would, if the 

 axis were to stand still ; the whole effect of the more rapid mo- 

 tion of the outer circumference being not to make the circum- 

 ference rotate more rapidly round the axis, but to make the axis 

 itself move forward so much faster than the inner point of cir- 

 cumference as virtually to describe a circle round that very point, 

 instead of that point revolving round the axis. And this is fully 

 confirmed by what has just been said in illustration of the converse 

 movements of the heavens and the earth. 



The variation in the length of the solar day is ascribed to the 

 difference of the sun's velocity in different parts of his orbit ; but 

 the only way in which such different degrees of velocity could 

 affect the length of the day, would be by making him, or more 

 correctly, the earth turn more rapidly, or more slowly, as the 

 case may be, round on her axis ; and thereby to make her rota- 

 tions not uniform, or of precisely the same length. In oppo- 

 sition, however, to this legitimate inference from the alleged 

 cause, astronomers say that the earth's rotation is precisely uni- 

 form, and is measured by the sidereal day, which, says Mr Rir- 

 low, " is the most uniform of all astronomical periods, neither 



