er if this will not satisfy, let us remember that the cause of its cohesion, which is indeed but the same medium, is always present to the body in every instant of its motion, and is equally adequate to the effect: if it were not present, its parts would be separated by the resistance of the air, and the cause of gravity would carry them to the earth. Upon the whole, motion is either a cause or an effect: it cannot be a cause, for this reason, that nothing can be the cause of itself; and if it is allowed to be an effect, then all the consequences will follow, as they are above laid down.

Causes may be inferred by Rational
Deduction.

Sometimes the bodily senses are able to discover the relation between the effect and its proper cause; and sometimes reason, grounding itself on past experience, will be able to do the same. When the mercurial fluid rises an inch higher to-day in the tube of a barometer than it stood yesterday, we neither see nor feel the cause of this alteration; but we come at it by very sure deduction, in the following manner: we discover,

by experiment, that the Torricellian vacuum is filled by the column of mercury, till the mercury becomes a counterbalance to the pressure of the external air; and that the air is the impelling cause, which drives the mercury up into the tube, and keeps it suspended there. Hence we conclude, that an alteration in the air has occasioned an alteration in the tube; and that the pressure of the atmosphere, being about one thirtieth part greater to-day than it was yesterday, drives the mercury so much higher, till it comes to an equilibrium. In the same way most of the other motions that are in the world may be referred to their proper causes, either by immediate experiment, or rational deduction. It does not follow that no material cause is concerned in any particular effect, only because we neither see nor feel its operation. There are many obvious cases in which the cause of motion may be assigned with certainty, though it is not perceived by any of the outward senses; and partly for this reason, because that which is manifest in some instances is occult in others, and distinguishable only by rational deduetion. When a thermometer is held before the fire, we discover, by its rising, that the

fire enters the pores both of the glass and the inclosed fluid; which being thereby expanded, and increased in its dimensions, necessarily rises higher in the tube. Here the

cause is sensibly known, because we feel the element of fire by its heat, and see it by its light. The like motion is observed in a lesser degree, when the thermometer is placed in the vacuum of an air pump, though no culinary fire is near, neither does the light of the sun act upon it; yet, though the thermometer rises only one or two degrees, a quantity too small for the human body to be sensible of*, we learn the cause of this in other more open experiments, and can thence infer with certainty, that fire is present in what we call a vacuum, to produce its natural effect of expansion; although it does not discover its presence to the senses of the body.

Different Sorts of Motion.

If we proceed next to the different kinds

*This is not accurately true under all circumstances; because the body, in a warm bath, will distinguish a difference of one of Fahrenheit's degrees, or even half a degree with practice, when the temperature of the bath is near the temperature of the blood; as I have found by experience.

of motion which are observed in the common course of nature, it will appear, that there is scarcely any such thing to be found as uniform motion in a right line. When a body

descends from the air toward the earth, its motion is not uniform, but continually accelerated; which must be the case with every body that is acted upon by an unremitting impression. The wheel which is moved by the impulse of a stream of water, begins to stir at first with a very slow motion, and proceeds to acquire new velocity, till the impediments are a balance to the impelling force: after which it moves uniformly, but with a velocity different from what it received at the first impression, and, as near as may be, when all interruptions are allowed for, to the velocity of the stream which gives it motion. The motions observable in nature are generally in curve lines. The wind blows with a serpentine motion, as may be discerned by tracing its impressions over a plain covered with snow, or over a field of standing corn. The water of the sea is driven into waves, which rise and fall alternately; the fire of lightning descends in crooked lines or zigzags: the clouds fly in a curve parallel to the surface of the earth: the planets move

in orbits, with a velocity not uniform in any one of them; projectiles are alternately retarded and accelerated, if their direction is above the level of the horizon. The light indeed does move in right lines, but whether its velocity is uniform at very great distances from its source, has not yet been ascertained.

Motion of the Parts of Fluids progressive and vibratory.

And here it may be seasonable to note, that fluid mediums are affected by two kinds of motion, either by an absolute progression of the parts proceeding continually forwards, as in the current of a river; or by a vibration propagated along them, without an actual progression. Of this latter sort is the motion of the sea before the wind; for it is not to be imagined, that the matter of the wave which you see a mile off, proceeds with the same succession as the figures of them are propagated from thence to the shore. The air in the form of wind moves progressively; but in the form of sound it is vibratory; it has an undulatory motion propagated through it without a progression of the parts: and what

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