of two miles an hour, and go against a wall, be will strike it with a sensible force or momentum ; if he walk at the rate of four miles an hour, and go against it, he will strike it with double the force; or if he run at the rate of six miles an hour, he will strike it with three times the momentum.

Obs.-Every child that throws a ball, or shoots a marble, is sensible that its force or momentum is in proportion to its velocity; the same marble will hit twice as hard, he will tell you, if it move twice as fast; or ten times as hard, if it move ten times as fast. Let him substitute the word momentum for hard, and velocity for fast, and he will more clearly understand the principle of the mechanical powers.

166. If a man, twice the weight of the boy, go with the same degree of swiftness, or with the same velocity as the boy, he will go against the wall with twice the momentum of the boy in every instance. Hence, it is a general rule, which must not be forgotten, that the momentum is always as the quantity of matter multiplied by the velocity with which the body moves.

Obs. A marble, twice the size of another, thrown with equal velocity, will strike with twice the force, and this is all that need be understood. Any one who has learnt the multiplication-table, may easily calculate forces or momenta: a ball of two pounds weight, moving with a velocity of six miles an hour, will strike with a momentum which may be represented by two multiplied by six, producing twelve; and a ball weighing six pounds, and moving at the rate of eight miles an hour, will have a momentum equal to six multiplied by eight, producing 48: hence, those two balls move with separate momenta equal to 12 and to 48; or, in similar terms, one moves with four times the force of the other.

167. If a stone weighing 500 pounds, is to be raised one foot by a man, who can lift only 100

pounds, he cannot raise it, unless he can contrive to make his arm move five feet, while the stone moves only one foot; because 100 multiplied by five, is equal to 500 multiplied by one.

168. This increase of motion in the arm is effected by the lever; because the motion of one end is in the same proportion to the motion of the other as the distance of the two ends are from the fulcrum.

If a lever, six yards in length, be laid on a fulcrum, at one yard from one end, and the above named stone be fixed to that end; the hand which pulls at the long, or five yards' end, moves over five times the space that the other end does; consequently, though pulling but 100 pounds, it will be equal to 500 pounds at the short end of the lever.

169. The grand principle then of mechanics is this, that we gain in power what we lose in motion; and hence, the strength of one man could move the earth, if he could bring his strength to act upon it with such a velocity, as there is difference betwixt his power and the weight of the earth.

Obs. The property of the simple lever is exemplified in the steelyard used by the butchers for weighing meat; and in the iron crow.

170. Single pulleys merely improve the purchase; but compound pulleys enable the hands of those who pull them, to move over twice the space according to the number of pulleys; hence, two acting pulleys increase the power four times, and three increase it six times..

171. A force applied to the circumference of a large wheel, as water, wind, and the feet, or

strength of men and horses, gains power in the proportion of the diameter of the wheel to the axle.

If a water wheel be 12 feet in diameter, and turn an axle of one foot, the powers acting at the circumference of the large wheel, moves over twelve times the space which the circumference of the axle moves; hence 12 cwt. may be raised with the power of one cwt.

Obs.-All windlasses, cranes, mills, windmills, and watermills, are formed on this principle. The power, whatever it be, is applied to the circumference of a large wheel, whose circumference moves in consequence, perhaps, ten miles an hour, while its axle, one-tenth of the diameter, moves but one mile an hour; consequently, the strength of one man at the circumference, will be equal to that of ten men at the axle.

172. Inclined planes, or sides of hills, wedges, screws, jacks, &c., are all used in mechanics on the same principle: their power depends on the proportion between the height actually attained, and the length of the plane moved over.

A screw is an inclined plane; and if a lever be added to it as it presses, the power gained is so great, that a man can multiply his own strength many thousand times.

Obs. If I wish to roll a cask weighing six hundred weight, on an elevation equal to ten feet, and my own force is but two hundred weight, it is obvious, on the above principle, that an inclined plane must be three times ten feet, or 30 feet long. If a mail-coach weighs vo tons, and is drawn on level ground, by a force equal eight hundred weight, and is to be drawn to the top a hill which rises twenty yards in a hundred, the rses will have to pull with an additional force equal to e-fifth of the weight of the carriage, i. e. one-fifth of 40 t., or double that with which they could draw on level Found.

F

173. A body put in motion, would move for ever, if it were not for the friction of the parts, and the resistance of the air, which alone stop it. A fourth of all power is lost in machinery, from Friction and Resistance: hence, the use of oil to soften the parts; the necessity of smooth roads for wheel-carriages; and hence, various contrivances called friction-wheels for diminishing friction.

Obs. 1.-The principle of bodies continuing in motion after being put into it, is felt by those who are in a carriage which suddenly stops. They are thrown forwards, owing to their not parting with the motion they have acquired. From this cause, when a ship in full sail strikes on a rock, every one on board is thrown down, and generally the masts snap in two; so when an open chaise stops from a horse falling, those in it are thrown forward; not from the position of the chaise, but owing to the motion which has been communicated to their bodies.

2.-Hence, also, rollers are very useful assistants in moving heavy stones or bodies, from the little friction they create.

174. The principle of all the mechanical powers, however they are combined, is the same; that is to say, to create all the difference possible between the velocity of the power, and the velocity of that body which is to be acted upon, so as to increase the momentum of the power.

One of the most common combinations is effected by cogged wheels; which, when turned by some power, move greater or smaller wheels, or give new directions to the force. !

175. A small wheel, with eight cogs or teeth, moving another which has forty cogs, diminishes the motion of the axle of the larger wheel a fifth, and increases the power five times, and this is the common windlass.

Some wheels are destined to effect certain objects without increase of power, as in clocks or watches. Sometimes, a greater power is applied to produce increased motion, as in the roasting jack, and in many mills.

Obs.-On duly considering the vast increase of power by some of these combinations, it cannot be matter of further wonder that first-rate men of war, and other such vast objects, are easily constructed. In some instances, one man is enabled to lift as much as 1000, and powers may be applied equal to the strength of a hundred thousand men.

176. Clocks and watches are nothing more than a simple arrangement of wheels of different diameters and numbers of teeth, so as to indicate seconds, minutes, hours, days, and even months.

Clocks are set in motion by a weight which turns a cylinder, which cylinder sets the whole in motion.

Watches are kept in motion by a coiled spring, which, in seeking to uncoil itself, exerts a power that gives motion to one wheel, which turns all the others.

Obs.-A lecture at a watch-maker's, with the wheels and other parts of a watch under the eye, will explain more than could be done by verbal description in a volume.

177. The triumph of mechanics is the steamengine. The inventor observed the excessive force of steam in lifting up the stiff lid of a kettle as he sat at breakfast, and he and others have since applied this resistless power to produce a motion applicable to all kinds of machinery.

178. In constructing these engines, steam from a copper is thrown into a hollow iron cylinder,