only as unreafonable, but as unneceffary; there being no occafion to confider motion as the cause of it's own continuation; for motion is only an effect, and muft, like all other effects, be referred to it's proper caufe. The nature of motion will be more fully treated hereafter: I fhall now only lay before you the fentiments of fome able men on the prefent law.

Dr. Horley, among others, confeffes it cannot be defended, and "believes, with the author of the Ancient Metaphyfics, that fome active principle is neceffary for the continuance, as well as for the beginning, of motion. I know," fays he, "that many Newtonians will not allow this: I believe they are misled, as I myself have been formerly mifled, by the expreffion a state of motion. Motion is a change, a continuation of motion is a further change; a further change is a repeated effect, a repeated effect requires a repeating caufe. State implies the contrary of change; and motion being change, a ftate of motion is a contradiction in

terms.

The property of bodies to perfevere in a state of rest or motion, which is announced by this law, is ufually termed their vis inertia, and fometimes, in cafe of a moving body, their vis infita, or inherent force. "These terms, and the ideas conveyed by them," fays Mr. Robinfon, profeffor of philofophy in Edinburgh, "are inaccurate, and fhould be ufed only to exprefs the neceffity we are under of employing force in order to produce a change in

motion.

M. Prony, in his "Architecture Hydraulique," fhews that these terms are equivocal, and only ferve to perplex and confufe the mind; that, ignorant of the nature of force, and the nature of mo

*Bishop of St. David's.

tion,

tion, we fhould confine ourselves to the effects alone.

Mr. Berrington. "This vis inertia, or power of refiftance, is generally faid to be fomething paffrue; fomething which refifts action, but does not itfelf act. However, either to refift is really to act, or else it is a word void of all meaning, and therefore ought to be exploded from a fcience which is not to be amufed with empty words.

LAW II. Every motion, or change of motion, in any body, must be proportional to, and in the direction of the force impreffed.

This feems nothing more, than that any change fhould be proportional to the cause producing fuch change; and, confidering motion as an effect, it will always be found, that a body receives it's motion in the fame direction with the cause that acts upon it. If the caufes of motions are various, and in different directions, the body acted upon muft take an oblique or compound direction, refolvable into two or more fimple directions; and hence it will follow, that the caufe of a curvilinear motion cannot poffibly be fimple, but must arise from the joint effect of different caufes concurring at the fame inftant to act upon the body.

LAW III. Re-action is always aqual to action, and contrary thereto; or, in other words, the actions of two bodies on each other are always equal, and are exerted in oppofite directions.

Action and re-allion are correlatives; one cannot exist without the other: refiftance, or re-action, is a neceffary condition to action: even action becomes re-action by a mere change of circumftances. In other words, a acting on b, as it determines b, is faid to act; as it refifts the re-action of b, itself re-acts: and confequently every body that acts, is at the fame inftant active and reactive.

VOL. III.

I

If

If you prefs with a finger one scale of a ballance, to keep it in equilibrio with a weight in the other scale, you will find that the fcale preffed by the finger acts against the finger with a force equal to that with which the other fcale endeavours to defcend. Thus, alfo, if a man in one boat, a, draws another boat, b, they will approach each other with equal quantities of motion. If the cord be cut in the middle, the part next a will fly back towards a, and that part nearest b will go to b which would not happen if the cord was only drawn towards a.

When a horfe or horfes are drawing a boat of barge against the ftream up the river, the rope which connects them to the boat, draws the horses as much as they draw the boat. In other words, whatever motion the horfes communicate to the barge or boat, they lofe an equal quantity themfelves: for fuppofe them to draw with a force equal to 100, and the force neceffary to keep the cord tort be equal to 50, they will not be able to draw with any more than the remaining force 50; for the fame force of mufcles and finews which they exert in order to drag the boat, would, if they were freed from the incumbrance, enable them to go much further than they can go, in the fame time, while connected with the boat.

When a cannon is difcharged, the rarified powder acts equally on the ball and the breach of the gun; for the rarified air, expanding itself every way, will equally prefs the cannon backwards, and the ball forwards, communicating the fame quantity of motion to each, though the velocities are very different. If the ball weighs rolb. and the cannon and carriage 10,000lb. the velocity of the ball will be 1000 times greater than that of the cannon, but the quantity of motion equal.

The re-action of the water on the oar occa

fións a boat to advance, and communicates to it as much motion as it received; the fishes perform by their tail and fins, and aquatic birds with their feet, what the waterman effects with his oar, the mutual action and re-action impelling them along.

Birds fupport themfelves, and pass through long tracts of air, notwithstanding the weight of their bodies much exceeds that of an equal volume of the fluid in which they move. If their wings ftrike and push the air towards the earth, it's reaction fupports their bodies; if the air be pushed towards the eaft, it's re-action drives the bird towards the weft. Birds which fly far, as the swallow, the falcon, &c. have generally fmall bodies and large wings, proper to act on a large quantity of air; while thofe whofe flight is fhort, and for a little time, have fmaller wings, and strike the air oftener than the others. If you compare the mufcles of a man's arm with thofe in the wing of a bird, you will find that the most robuft and agile man would never be able to move wings proportioned to the weight of his body, with a velocity capable of fupporting him in the air.

Daniel Bernouilli propofes a new way to navigate veffels, founded on this principle of action and re-action. He requires a tube to be fixed to the ftern of a veffel, open at both ends, and kept conftantly filled with water, by means of a pump. The water running out of the tube would act against that in which the veffel lay; the re-action of this would push the veffel forward without fail or oar. Jacquier tried the experiment on a small fcale with fuccefs.

OF COMPOUND MOTION.

By compound motion, I mean that which is produced by the fimultaneous action of feveral powers acting in different directions.

The compofition is not in the motion, but in the powers or forces by which it is produced.

General law of compound motion. Any body folicited to move by the fimultaneous action of different forces acting upon it in different directions, will take a mean direction between what each power tends to communicate to it, and will move with a velocity proportionable to the force acting efficacioufly for as the body cannot move in more directions than one at the fame time, it must move in a direction refulting from the combination of the acting powers; and the intenfity of the force it receives, will be as the intensity of the force fubfifting in cach, independent of their oppofitions.

1. Thus, if two forces act at the fame time on any body, and in the fame direction, the body will move with a velocity equal to the fum of the velocity of the two forces acting upon it.

2. If the two forces are equal, and act in exactly contrary directions, the body will not move; as the equal and oppofite forces of the acting body deftroy each other.

3. If the forces are unequal, but acting in oppofite directions, the body will move in the direction of the firongeft, with a velocity equal to the difference of the velocities in the two given bodies.

PROPOSITION. A body alied upon at the fame time, by two forces whofe directions and intenfities are reprefented by the fides of a parallelogram, will defcribe the diagonal in the fame time it would have defcribed