442 
only applies the illustration to the ease of the earth and planets, as revolving 
round the sun at rest. Adopting this illustration quantum valeat, let us now 
apply it to the motion of the moon round the earth as a moving centre. We 
have now only to suppose that our larger steamer also gets up steam, and 
begins to move eastward, say with a speed of twelve knots an hour, and 
watch the result. The little steamer being detached in one sense from the 
other — though it is attached to it in a different sense, i.e., held by a rope at 
a certain distance detached from it — does not partake of the motion of the 
larger vessel, as all bodies in its cabin and on its deck do, as in Ferguson’s 
previous illustration (§ 18, ante). The little vessel, therefore, now falls 
behind, where it will be towed along ; the only effect of its exerting its steam 
power of two knots an hour being to lessen, pro tanto , the tension upon the 
rope that holds it. In order that the small vessel may now go round the 
other as before, and keep the rope always stretched out with the same 
tension, while the larger vessel now steams along at twelve knots an hour, it 
will require a horse-power sufficient to give it a speed of fourteen knots an 
hour in moving eastward, and when it has passed before and round to the 
other side of the larger vessel, it must then have reduced its speed to ten 
knots an hour, still however steaming in an easterly direction, or it could not 
make its apparent revolution round the other. 
24. This illustration, however, would only be strictly analogous if the 
moon’s motion in its apparent circular orbit were always the same ; which is 
not the case. If that were so, then the influence of the sun upon the moon’s 
motion would be omitted as imperceptible or nil. according to the usual 
methods of dealing with this problem ; for it should be remembered that it 
is what is called “ the moon's variation ” (that is, the variation of her motions 
in her apparent orbit) that is attributed to the influence of the sun’s attrac- 
tion. Let us, therefore, leave illustrations, to reason from the actual facts of 
the case that is under discussion, which are perfectly clear of themselves, and 
really not in dispute. I admit the apparent increase of velocity in the motion 
of the moon as she approaches the sun ; and, were this apparent increase of 
velocity real, instead of merely apparent, I would further admit that it might 
be caused by the sun’s attraction ; but what I maintain is, that if we believe 
in the Copernican theory, we also know quite well that this apparent increase 
of velocity as the moon approaches the sun is only apparent and unreal, 
being, in fact, the result of the moon’s decreasing velocity when viewed from 
the earth as a stand-point. As the earth is, then, ex hypothesis moving quicker 
than the moon, the moon merely appears to move quicker, and also to move 
in an opposite direction, contrary to reality, as we have seen is also the case 
in the simple illustration of the passing railway-trains. If we really believe 
the earth to be in motion, then we have only to take into account its velocity 
eastward, in order that the apparently increasing motion of the moon the 
other way may be known to be, in fact, a decreasing velocity in the same 
eastward direction ; and, consequently, if this variation of the moon’s velocity 
is attributable to the influence of the sun, it follows that that influence must 
be repulsive , since it has really retarded the moon’s velocity in approaching 
the sun. In like manner also, therefore, as the moon’s motion, which is 
apparently retarded and decreasing from her place in conjunction till in her 
first quarter, is really increasing during that time, and goes on increasing 
more and more as she recedes from the sun till she reaches her greatest dis- 
tance in opposition, the real influence of the sun upon the moon must be 
repulsive , or, the reverse of that, attributed to the sun, when only the appa- 
rent variations of her motion are considered, instead of the real variations, 
upon the Copernican hypothesis. 
25. How this obvious oversight can have occurred is not the question. 
To say the least, it is certainly remarkable, when we consider that the very 
