ufe of emeralds to affift their fight, which were made concave, the better to collect the rays, (concavi ut vifum colligant ;) and that Nero ufed them in viewing the combats of gladiators, xxxvii. 5. f. 16. Seneca fays, that the smallest letters, which could hardly be difcerned with the naked eye, might be dif tinctly read through a glafs tube filled with water; as the ftars appear larger when viewed through a cloud; Quaft. Nat. i. 6.; and Aulus Gellius fpeaks not only of multiplying mirrors, but also of fuch as made the objects appear inverted; xvi.

18.

The magnifying and burning power of glaffes is fuppofed alfo to have been known to the Druids, from certain lenticular or spherical gems of rock cryftal, belonging to them, as it is thought, which are still preferved in the cabinets of the cu

rious.

The doctrine of the refraction and reflection of light has been wonderfully illuftrated in modern times, by the various inventions and improvements of glaffes. Of these the moft remarkable are the telescope and microscope, both refracting and reflecting.

The first telescope was made by Zachary Janfen, a maker of fpectacles at Middleburgh, in 1590. Galileo, profeffor of mathematics at Padua, having heard of this difcovery in 1609, but without feeing any fuch inftrument, fet himself to contrive one of the fame kind; in which he fucceeded, and in a fhort time carried his improvements to a furprifing degree of perfection; See p. 29. But it was Kepler who firft explained the reasons of the effects of telescopes.

The reflecting telescope was invented by Mr James Gregory of Aberdeen, and improved by Sir Ifaac Newton. But the greatest improvement on telescopes of the dioptric kind, was made fome years ago by Mr Dollond; who ingeniously contrived a method of correcting the defects of former inftruments of this kind, arifing from the different refrangibility of the rays of light, by the application of two different kinds of glass, poffeffed of different powers of refraction, and of different figures, fo that the effects of the one exactly counterbalance those of the other.

In a reflecting telescope we never fee the object itfelf, but only that image of it which is formed next the eye in the telefcope.

The magnifying power of the reflecting telescope was confiderably increased by Mr Short, and has been lately augmented to a wonderful degree by Dr Herschel.

I 2

The

The microfcope was invented foon after the telescope by Janfen and his fon, improved chiefly by Leuwenhoek, a Dutch phyfician, who was born at Delft in 1632, and died 1723. The microfcope has as much extended the fphere of human knowledge, with refpect to the nature and properties of minute fubilances, as the telescope in viewing diftant objects.

Of MOTION, and its LAWS.

Motion is the removal of a body from one place to another, or a continual change of place.

Any force acting upon a body to move it is called a Power. The momentum or quantity of motion is in proportion to the force impreffed. The heavier any body is, the greater is the power required either to move it, or to ftop its motion. fcience which teaches the effects of powers or moving forces, in as far as they are applied to engines, is called MECHANICS.

That

The fimple machines employed to move bodies are called mechanical powers. Thefe are fix in number, the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the forew.

1. The LEVER, a ftraight bar of iron or other fubftance, fupported on a fulcrum or prop, made ufe of to raise weights to a fmall height, is of two kinds; 1. when the weight to be raifed is on one end, the force applied on the other, and the prop between both, but much nearer to the weight than the power, which is the common fort of lever. 2. When the prop is at one end, the ftrength applied to the other, and the weight between them. Thus in raifing a water plug in the ftrects, the workman puts an iron lever through the hole of the plug, till it reach the ground, and making that his prop, lifts the plug with his ftrength at the other end of the lever. Our common balance is a common lever; fo alfo the Roman fteel-yard, (ftatera vel trutina); the arms (brachia) of which, or the two ends of the beam, are poifed on a prop, an axis or cen-` tre, whereon it turns. When the weights on each brachium are equal, the balance is faid to be in aquilibrio, or the weights to equiponderate. Unequal weights hanging at different diftances from the centre may equiponderate.

2. A wheel turning together with its axis, hence called AXIS IN PERITROCHIO, which ferves to raife weights to a greater height. The power in this machine is applied to the circumference of the wheel, by the motion of which, a rope that

is tied to the weight, is wound about the axis, by which the weight is raifed.

3. The PULLEY, or a little wheel moveable about its axis with a rope running over it. A machine made by combining feveral pullies together, is often made ufe of to raife weights, when the axis in peritrochio cannot be applied.

4. The INCLINED PLANE, the advantage gained by which is as great as its length exceeds its perpendicular height. Thus if the plane be three times as long as high, a weight may be rolled up on it with a third part of the power which would be requifite to draw up the fame weight perpendicularly.

5. The WEDGE; which may be confidered as two equally inclined planes joined together,

6. The SCREW; which cannot properly be called a fimple power, because it is never ufed without the application of a lever, or winch, to affift in turning it; and then it becomes a compound engine of very great force, either for preffing the parts of bodies, or for raifing great weights.

Various machines are conftructed, in which these fimple mechanic powers are all combined; hence called compound machines. We may judge of the knowledge of the ancients in mechanics from the ftupendous works which they reared; efpecially from their moveable towers, and other warlike engines. A thing of the greateft importance in mechanics is to difcover the centre of gravity of bodies.

The CENTRE OF GRAVITY in a body is a point where all the parts of the body, in whatever pofition it is, are in aquilibrio. When the centre of gravity is fuftained, the body remains at reft. When feveral bodies are joined together, that point on which the whole may be poifed, is the centre of gravity. Hence a building will stand, although confiderably bent from the perpendicular, while its centre of gravity is fupported: as the famous tower of Pifa, which inclines feventeen feet; fo at Bologna, &c. Whatever point in a body or machine fuftains the centre of gravity, fuftains the whole weight; fo that the force with which any body tends towards the earth, is, as it were, collected in that centre.

The laws of motion, eftablished by Sir Ifaac Newton, which he calls the Laws of Nature, are three in number.

1. All bodies continue in the ftate they are in, whether of reft or motion, till they are obliged to alter that state by some force impreffed. All motion is naturally rectilineal or in a ftraight line

2. The change of motion is always proportional to the moving force impreffed, and is always made in the fame direction with the impreffing force.

3. Action is always equal and contrary to re-action. When one body ftrikes against another, both fuffer equally. Thus a loadstone draws iron, and is equally drawn by iron. The weight of the carriage pulls back the horse, as much as the horse pulls it forward.

That science which explains the laws of nature and the properties of body is called NATURAL PHILOSOPHY.

The force of gravity decreases as the fquare of the distance increafes, or as the diftance multiplied by itself increases; that is, a body at twice the distance of another attracts with only a fourth part of the force; at thrice the distance, with a ninth part; at four times the diftance, with a fixteenth part; and so

on.

The velocity of falling bodies is in proportion to the times of their defcent; thus, if a body move one mile the first minute, it will move three the fecond, five the third, feven the fourth, nine the fifth, and fo on. Hence the whole spaces are as the fquares of the times; thus in two minutes the body will have fallen four miles; in three, nine; and in four, fixteen; and accordingly it is found that a body falls about fixteen feet in one fecond of time; in two feconds, four times fixteen, or fixtyfour feet; in three feconds, nine times fixteen, or 144 feet, &c.

When a body moves by two joint forces, not in the fame direction, and uniformly by both, it will move in a straight line, and will defcribe the diagonal of a parallelogram, in the fame time that it would defcribe either of the fides by one of the forces fingly. But if one of the forces act in fuch a manner as to make the body move faster and faster as it goes forward, which is called an accelerative force, then the line defcribed will be a curve. And this is the cafe of all bodies projected in a rectilineal direction, and at the fame time acted upon by the power of gravity. The curves which bodies defcribe are different in different circumftances. The curve which a ftone projected from the surface of the earth describes, is called a Parabola; produced by the joint effect of the projectile force, or the force with which it was thrown, and the force of gravity, by which it is every moment drawn to the earth.

In like manner, the circular figure described by a body moving round a centre is formed by the joint action of the centripetal

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