Слике страница
PDF
ePub

[61] ofte vw-X. Of the Mechanical Porders.

**+73. Without the aid of art, man could not Praise Nassy stones to the tops of churches and palaces: he could not apply immense beams of timber to his purposes ; in short, he would still have been a builder of huts and cottages. He would, however, soon discover the use of a lever, and the principle of that power would soon be applied, in various shapes.

Obs. -A lever is the foundation of all the mechanical powers." It is nothing more than a straight stick or bar of wonit or n'on; and any common lever may be applied in an instant to any object by way of experiment : -a pokera fire-shovel, or strong walking-stick, for the purpose of illustratiou, is as good a lever an any that could be inade. Lay a shovel across a fender, and put a large coal into it, theo balance the conl with the hand at the other end of the shovel ; in this situation, the shovel is u lever, the fender is the fulcrum, the coal is the weight or resistance, and the hand is the power to raise and overcome it.

174, The force with which any body moves is called its momentum. If a boy walk at the rate of two nules an hour, and go against a wall, he 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 hall, or shoots & marble, is sensible that its force or momentum'is in propor. tion to its velocity ; the same marble will hit twice as bard, he will tell you, if it move twice as fast, or ten times as bard, if it more ten times as fast. Let him

[ocr errors]

substleute the word momentum for hard, and vanity for ful, and he will at onee understand the prtociple of the puechanical powers,

175. If a hll, twice the weight of the boy, go with the same degree of swiftness, or with the hume velocity as the boy, he will go aguinst the wall with twier the momentum of the boy in every instagee. Hence, It is a general rule, which must not be forgotten, that the momentum is always in proportion tu the combined or united size and velocity of the forces enıployed,

Obo- A marble, twire the size of another, thrown with equat veloeity, will strike with twice the foree, and this is all that need be understool. Any one who has learned the multiplication title, my pasily coleulule forere or momenta , a ball of Iwo pounds well, mo. dug with a veloeity of six miles an hour, will alrike with A momentum which may be reprezentei by to multi. plied by sia, producing twelve 1 and a hall weighing sit pounds, and moving at the rate of eight miles in hour, will have a momentum equal to ex multiplied by righe, producing 48 henee, those two bulls move with be par le momenta equal to 1e a do AM, or, in simpler terint, une moves with four times the force of the other,

170. If'a stone weighing 600 pounds, is to be raised one foot by a man, who can litt only 100 pounds, be ennnot raise it, unless lie can com Trive to make his arm move five feet, while the stone moves ouly one foot: because 100 multiplied by five, is equal to 600 inultiplied lay one,

177. This inerende of motion hi the star is effected by the lroer, because the motion of one end is in the sante proportion to the motion of the other, un tlie distance of the two ends are from the futerum.

11 « lever, six yards in length, laid on u Yul.

crum, at one yard from one end, and the abovenamed stone be fixed to that end; the hand which pulis 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.

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

Obs. The property of the simple lever is exemplified in the steelyard used by butchers for weigbing meat , and in the iron crow. "179. Single pulleys merely improve the purcbase; but compound pulleys enable the hands of those who pull then, 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.

180. 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 power acting at the circunference of the large wheel, move over twelve times the space which tho circuniference of the axle moves; hence 12 cwt. may be raised with the power of one cwta?

TOM. MIT wind lammasid eranew, mille' windmill;' *** watermills, are formed on this principle. The power whatever it be in applied to the circumference of a large wheel, whose circumference mover in consequence, per hape, ten miles an hour, while its axle, one iemide of the diameter, noves but one mile an hour a consequently, the strength of one man at the circumference will be equal to that of ten men at the axle.

181. Inclined planes, or sides of bills, wedges, screws, jaeks, &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 in presses, the power gained is so great, that a man can multiply his own strength many thousand tines,

Oas.-- If I wish to roll * eask, weighing six hundred weight, on an elevation equal to tey feet, and my own force is but two hundred weight, it is obvious, on the above principle, that an inclined pane must try three timos ten Moel, or 80 feet long. If mail-coach weighs two tons, and is drawn on level ground, by a forcerqual to eight hundred weight, and is to be drawn to the top of a hill which risen twenty yards in a moudrod, the horses will have to pull with an additional force equal to onefinth of the weight of the carriage. I ei ene fifth of 10 ewt., or double that with which they could draw on level

ground,

182. 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 smooth the parts; the neocasity of smootha ronda for wheel-carriayen , and liance,

1

various contrivances called friction-wheels, for diminishing friction.

006. -The principle of bodies continuing in motion after being put into it, is felt by those who are in a car. ringe which suddenly stops. They are throwo forwarde, owing to their not parting with the motion they have noquired. From this cause, when a ship in full sa il strike on a rock, every one on board is thrown down, and generally the masts vnap in two so when no oprv chaise stops from a horse falling, those in it are throwu Torward ; not from the position of the chaise, but owing to the modion which has been commonleuted to their bodies.

9.--Hence also, rollers are very useful assistante in anoving heavy stones, or bodies, from the little friction they create.

183. The principle of all the mechanical powers, however they may be combined, is the same; that is to buy, to create all the difference possible betroeen the relocity of the percer, and the velocity of that body which is io be acted upon, so as to increase the momentum of the

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

184. A small wheel, with right 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 roastingjack, and in many mills.

« ПретходнаНастави »