NATIVITY

, in Astrology, the scheme or figure of the heavens, and particularly of the twelve houses, at the moment when a person was born; called also the Horoscope.

To Cast the Nativity, is to calculate the position of the heavens, and erect the figure of them for the time of birth.

NATURAL Day, Year, &c. See Day, Year, &c.

Natural Horizon, is the sensible or physical horizon.

Natural Magic, is that which only makes use of natural causes; such as the Treatise of J. Bapt. Porta, Magia Naturalia.

Natural Philosophy, otherwise called Physics, is that science which considers the powers of nature, the properties of natural bodies, and their actions upon one another.

Laws of Nature, are certain axioms, or general rules, of motion and rest, observed by natural bod es in their actions upon one another. Of these Laws, Sir I. Newton has established three:

1st Law.—That every body perseveres in the same state, either of rest, or uniform rectilinear motion; unless it is compelled to change that state by the action of some foreign force or agent. Thus, projectiles persevere in their motions, except so far as they are| retarded by the resistance of the air, and the action of gravity: and thus a top, once set up in motion, only ceases to turn round, because it is resisted by the air, and by the friction of the plane upon which it moves. Thus also the larger bodies of the planets and comets preserve their progressive and circular motions a long time undiminished, in regions void of all sensible resistance.—As body is passive in receiving its motion, and the direction of its motion, so it retains them, or perseveres in them, without any change, till it be acted upon by something external.

2d Law.—The Motion, or Change of Motion, is always proportional to the moving force by which it is produced, and in the direction of the right line in which that force is impressed. If a certain force produce a certain motion, a double force will produce double the motion, a triple force triple the motion, and so on. And this motion, since it is always directed to the same point with the generating force, if the body were in motion before, is either to be added to it, as where the motions conspire; or subtracted from it, as when they are opposite; or combined obliquely, when oblique: being always compounded with it according to the determination of each.

3d Law.—Re-action is always contrary, and equal to action; or the actions of two bodies upon one another, are always mutually equal, and directed contrary ways; and are to be estimated always in the same right line. Thus, whatever body presses or draws another, is equally pressed or drawn by it. So, if I press a stone with my finger, the finger is equally pressed by the stone: if a horse draw a weight forward by a rope, the horse is equally opposed or drawn back towards the weight; the equal tension or stretch of the rope hindering the progress of the one, as it promotes that of the other. Again, if any body, by striking on another, do in any manner change its motion, it will itself, by means of the other, undergo also an equal change in its own motion, by reason of the equality of the pressure. When two bodies meet, each endeavours to persevere in its state, and resists any change: and because the change which is produced in either may be equally measured by the action which it excites upon the other, or by the resistance which it meets with from it, it follows that the changes produced in the motions of each are equal, but are made in contrary directions: the one acquires no new force but what the other loses in the same direction; nor does this last lose any force but what the other acquires; and hence, though by their collisions, motion passes from the one to the other, yet the sum of their motions, estimated in a given direction, is preserved the same, and is unalterable by their mutual actions upon each other. In these actions the changes are equal; not those, we mean, of the velocities, but those of the motions, or momentums; the bodies being supposed free from any other impediments. For the changes of velocities, which are likewise made contrary ways, inasmuch as the motions are equally changed, are reciprocally proportional to the bodies or masses.

This law obtains also in attractions.