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described as being “ common to all,” while they may move among one another 
from other causes, they could scarcely have relied upon it as furnishing an 
argument applicable to the relative motions of detached and independent 
bodies like the earth and moon. All bodies in the ship are somehow attached 
to it, whether they stand or move about, while they are supported from below, 
or whether they hang and are swung about while they are supported or sus- 
pended from above. They partake of the common motion of the ship, because 
they are attached to it mechanically. Therefore, let us vary the illustration ; 
for it is a fact that men have been influenced by popular arguments upon this 
question much more than at first may be supposed. 
20. Let us suppose, then, that we are in one of the carriages of a railway- 
train, travelling eastward at the rate of 40 miles an hour, and that we over- 
take another train on a parallel line of rail moving also eastward at 35 miles 
an hour. When we pass that train, it will lag behind ours, and so it will 
appear to move away in an opposite direction. But would we, therefore, be 
entitled to reason as if the other train were really moving westward at the 
rate of five miles an hour ; and — which follows as a necessary consequence of 
our doing so — to speak as if our own train were at rest, though we know the 
facts to be that both are travelling eastward, only that ours is moving quickest, 
with a velocity of 5 miles an hour greater than the other ? Observe the result 
if we do. The train that is moving fastest must be regarded as not moving 
at all, and the other as moving in a direction opposite to its actual motion ; 
and then, also, the slower it moves, the greater its velocity will appear : the 
direction of the motion and the rate of velocity being both reversed and made 
contrary to reality. 
21. Or, again, were we in the slower train, would it be rational to speak 
of the greater velocity of the passing train as only a speed of five miles an 
hour, and regard our own train as at rest ? In that case, it will be observed, 
the mere appearance is not so utterly contrary to fact as in the other. As it 
is now the slowest train that is considered at rest, the direction of the 
motion of the other is not reversed ; the delusion is limited to the rate of 
the velocity. 
22. But, if it would be absurd to do this, when we merely consider the 
directions of the motions and the relative rates of velocity of the two trains, 
it would, if possible, be even more absurd, if we were further to reason from 
the mere appearances, as to the probable motive power, or force, which had 
produced the motions and respective velocities of the two trains. If the con- 
version, for instance, of one ton of fuel into heat represented the force that 
would produce a speed of one mile an hour for a given time ; and in like 
manner thirty-five tons a speed of thirty-five miles, and forty tons of forty 
miles an hour ; then, founding our calculations upon the mere appearances, 
instead of the real motions, would lead us to astounding conclusions. 
23. One other illustration will suffice. Suppose there is a steam-vessel 
with a single mast, floating at rest on a placid sheet of water, without any 
current and with no wind ; and at a little distance, that another smaller 
steamer is lying parallel and attached to the larger vessel by a rope looped 
round its mast. Then let us suppose the small steamer gets up steam, and 
begins to move eastward with a horse-power equal to propel it two knots an 
hour. The result will be that the small vessel will not steam forward in a 
straight line, but it will move round the larger vessel to which it is held 
attached by the rope. Although the rope does not draw the smaller vessel 
towards the larger, yet as it holds it at the distance of the rope’s length, and 
so causes it to move round, — an illustration like this has been often used as 
representing the revolving of a body held to a centre by gravity. Ferguson, 
among others, does so, in § 107 of his Astronomy : his illustration being a 
boat rowed by a man while attached to a stationary ship by a rope. But he 
