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motion of the body arises from the motion of the medium, then the resisting nature of the medium is no longer any objection to the motion of the body; neither can it be, for it is the cause of the motion ; and it would be absurd to imagine, that the cause of motion can resist the motion which it causes. Hence it is plain, that no inferences from the resistances of mediums can lead us to the necessity of a vacuum. We want a vacuum only when we propose a motion which is independent of the action of every medium: but nature has no such motion, and the whole affair of resistance is therefore of little use but to practical mechanics; because the motions of nature are otherwise circumstanced.
If there is any medium more subtile than air, such as we understand by ether, that medium is the cause of gravity; and should we make experiments in order to discover whether any medium besides the air resists a falling body, what is this but to inquire, whether the cause of gravity resists the motion which it causes? Wheresoever we contradict the motions of nature, we are necessarily resisted, let the case be what it will. if we endeavour to move any body parallel to the horizon, we are resisted by the cause
of gravity, which gives it a determination toward the centre of the earth; if it is descending in the perpendicular, and we give it a stroke downwards in the direction of its descent, we are still resisted in a proper degree; because, though we do not counteratt its direction, we contradict its velocity, and therefore the cause of gravity will resist as before. The inquiry how matter resists per se, in an independent state, is impracticable; because no matter, subject to our inspection, is in that state.
In the preceding Discourse, these things have appeared to us, which it may
proper here to recapitulate:
1. That the heavens, the earth, the elements, and the bodies contained in them, are constantly in motion; which indeed is a fact so obvious, that it needs no particular proof. • 2. That God is the primary cause of all this motion, but not the immediatè cause; it being the constant method of Divine Providence to work with intermediate or secondary
causes, and to move some bodies by the instrumentality of others.
3. That motion is never to be considered as a thing by itself, but as an effect; which, like all other effects, must be referred to its proper causes : for the parts of the world have motion, not of themselves, but as the limbs have, by means of their connexion with the body to which they belong. Whence it follows, that any parcel of matter taken independently, can have no more motion belonging to it than a limb divided from the body.
4. That motion is so far analagous to life, that they both require the permanent action of some cause to maintain them. As life is constantly preserved in an animal body by the causes of life, so is motion as constantly preserved by the causes of motion permanently acting for this purpose: and that to suppose any thing else, is to open a door for the doctrines of Atheism.
5. That the first law of motion, invented by Descartes, is not only unreasonable, but unnecessary as an expedient; there being no occasion to understand motion as the cause of its own continuation, when there is an active medium adequate to all the effects of gravity and projection; which, moving with infinite freedom, can so far deceive us by the subtilty of its vibrations, as to make us believe there is no cause, where the most powerful of all secondary causes is present: and if motion be not a quality in bodies, but a mere effect upon them, this reasoning is necessary and natural, especially as we can justify the practice of assigning physical causes by rational deduction, where they are not manifest in our experiments.
6. That uniform motion in a right line is a phænomenon scarcely observable in nature: for that the elements and natural bodies are wont to move, either in curve lines, or with velocities not uniform. The rays of light have a rectilinear motion, but we cannot be assured their progress is uniform, it being more probable that they are retarded in a certain proportion, as they proceed to very great distances from the sun.
7. That, in accounting for motion, the same cause may be applied to different effects, because the elements are capable of such different motions as will answer many purposes at once. If this is found in the grosser fluids of water and air, much more is it true in ether, light, or fire, which is capable of all directions, and may cross in a thousand different ways without interruption from itself: fire may heat a body, a candle may illuininate it, magnetism may give it a polar direction, electricity may repel it, and the cause of gravity give it a tendency toward the earth ; and all these may be taking effect upon it at once.
8. That as all motion is in the direction of its cause, there can be no such thing as a power of attraction ; because that supposes a direction in the cause which is contrary to the direction of the effect, and therefore can never happen consistently with the laws of mechanism. A cause from the earth can never bring the moon nearer to the earth; neither can a cause from the moon bring the waters of the earth nearer to that. The cause which is in the direction of a body that moves freely, is an impelling cause; and if it is impulse, it is not attraction; if it is attraction, it is not impulse; these principles being of opposite natures, and consequently inconsistent. . 9. That if the same medium is found to act in different or contrary directions, it must be supposed to be in different condi