414 
the result. Then we may suppose a watch placed upon a table, and suppose 
it to be placed on a revolving table which happened to revolve in a contrary 
direction to the motion of the hand of the watch. What would be the 
result ? The hand of the watch would remain in the same direction. We 
should not, therefore, interfere with the motion of the watch in going. And 
if we wanted to consider the relation of the motion of the hand to the 
machinery that drives it, we should not take into consideration the compound 
motions by which the hand appears to remain at rest ; but we should simply 
consider the motion of the hand in obedience to the mechanical force of the 
watch which drives it. With regard to Newton’s principle of circular or 
elliptic orbits, the same observation will apply. It is perfectly true that the 
moon does not describe an orbit, circular or elliptic, but describes a wavy 
line round the earth’s orbit round the sun ; but in considering that orbit the 
difficulties are simplified by considering the earth to be at rest. And it 
appears as the result of observation that the results deduced from such a 
supposition are not vitiated by the circumstance of the earth being in motion. 
Mr. Reddie alluded in his paper to motion in a resisting medium, and be 
referred to the confusion which would arise from such a supposition. Now 
the fact is, that the resistance of the medium which is supposed to pervade 
all space is the means of transferring the vibrations of light and heat from 
the sun and other sources to us, more especially from the sun to the earth. 
That it is a resisting medium has been proved by the retardation of Encke’s 
comet ; but the reason why it affected Encke’s comet was that it consisted 
only of a mass of vapour, and was so light and attenuated as to feel the 
least resistance. The earth or planets, being immeasurably heavier, are not 
affected at all. The motion of the earth in its orbit I believe to be wholly 
unaffected by the existence of ether, that elastic medium which pervades all 
space. Not because there is no resistance, but because it is so minute in 
comparison to the magnitude and weight of the body in motion. I might 
illustrate it in this way. If you exercise the slightest puff of breath upon an 
air-ball that a child plays with, you alter its course ; but would that afford 
any ground for supposing that, if you had the courage to stand near the 
mouth of a cannon, say a six-hundred pounder, from which a shot was about 
to be fired, and the moment the shot was passing out you gave it a puff, you 
would have any effect in altering the point at which the ball would strike ? 
Certainly not ; and the reason is precisely the same : the force of your 
breath in the one case and the resistance of the assumed medium in the 
other were so minute in relation to the mass in motion as to have no sensible 
effect 
Mr. Reddie. — I have granted quite as much as that in my paper, and 
reduced my objections to the effect of a resisting medium upon the earth’s 
atmosphere, (§ 27, last 18 lines.) 
Mr. Brooke. — I certainly understood that the gist of Mr. Reddie’s argu- 
ment was the assumption that there was a contradiction. He quotes the 
letter of Voltaire “I left the world full in Paris, but found it empty in 
London. In France the earth is believed to be shaped like a melon, but 
