With joy ambition saw, and soon improv'd X. Of the Mechanical Powers. 180. Without the aid of art, man could not raise massy 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 pow. ers. It is nothing more than a straight stick or bar of wood or iron; and any common lever may be applied in an instant to any object by way of experiment:--a poker, a fire shovel, or strong walking stick, for the purpose of illustration, is as good a lever as any that could be made. Lay a shovel across a fender, and put a large coal into it, then ballance the coal with the hand at the other end of the shovel; in this situation, the shovel is a lever, the fender is the fultrum, the coal is the weight or resistance, and the band is the power to raise and overcome it. 181. The force with which any body moves is call ed its momentum. If a boy walk at the rate of two miles 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 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, be 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 at once understand the principle of the mecbanical powers. 182. 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 is not to be forgotten, that the momentum is always in proportion to the combined or united size and velocity of the forces employed. Obs.--d 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 learned the multiplication table, may easily calculate forces or momenta : a ball of two pounds weight, inoving 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 simpler terms, one moves with four times the force of the other. 183. If a stone weighing 500 pounds, is to be raised one foot by a man, who can list 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. 184. 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, 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. 185. 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 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 weighing meat; and in the iron crow. 186. Single pulleys merely improve the purchase; but compounded pullies enable the hands of those who pull them, to move over twice the space according to the number of pullies.; hence, two acting pulleys inerease the power four times, and three increase it six times. 187. 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 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, wbatever 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. 188. Inclined planes, or sides of hills, wedges, screws, jacks, &c. are all used in mechanicks 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 is 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 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 two tons, and is drawn on level ground, by a force equal to eight hundred weight, and is to be drawn to the top of a bill which rises twenty yards in a hundred, the horses will bave to pull with an additional force equal to one fifth of the weight of the carriage, i. e. one fifth of 40 cwt. or double that with which they could draw on level ground. 189. 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 stops it. A fourth of all power is lost in machinery, from friction and resistance, hence, the use of oil to smooth the parts ; the necessity of smooth roads for wheel carriages ; and hence, various contrivances called friction wheels, for diminishing friction. Obs. 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. 190. The principle of all the mechanical powers, however they may be 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. 191. 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 tbe 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 ships of war and other such vast objects, are easily constructed. In some instances, one man is enabled to lift up as much as 1,000, and powers may be applied equal to the strength of a hundred thousand men. 192. Clocks and watches are nothing more than a simple arrangement of wheels of different diameters and number of teeth, so as to indicate seconds, minutes, hours, days, and even moons. Clocks are set in motion by a weight which turns a cylinder, and this cylinder setts 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 of ten minutes at a watchmaker's, with the wheels and other parts under the eye, will explain more than could be done by verbal description in a volume. |