Part 264

MACHINES used in war by the ancients. Every species of instrument or machine, which was employed before the invention of fire-arms, for the purpose of demolishing the fortifications of an enemy, or of rendering them accessible to the besieger, came under the denomination of machine. For a full and elaborate explanation of the different machines that were adopted by the ancients, we refer our military readers to the second volume of the Recueil Alphabetique, page 73.

MACHINES Infernales, Fr. Infernal machines. Although the first idea of these machines has been attributed to France, the invention, nevertheless, is by no means new. Frederic Jambelli, an Italian engineer, was the first that used them, when Alexander, of Parma, besieged Antwerp. The prince of Orange likewise had recourse to the destructive effects of an infernal machine, in order to bombard Havre-de-Grace, and to set it on fire. The Dutch and English, in conjunction, attempted to destroy St. Malo by the same means. The first instance, however, upon record, in which the French made use of this machine, was when Louis the XIVth ordered a vessel, carrying an enormous shell, full of every species of combustible matter, to be dispatched to Algiers, for the purpose of demolishing its harbor. This, the English say, suggested to other nations the adoption of fire-ships, and other destructive machines, which have frequently been used against maritime places, although they had been in use a century before.

The author of Oeuvres Militaires, tom. xxii. page 222, speaking of the infernal machines, observes, that if he were to be in a situation which required the use of so dreadful an explosion, especially to destroy a bridge, he would prefer having the machine made simply with different strong pieces of wood joined together, so as to be in the shape of an egg, or of a cone reversed. The whole must then be made compact with cords twisted round it. This method, in his opinion, is not only the best, but can be executed in the most easy and expeditious manner. He further adds, that in order to burn and blow up wooden bridges, and even to destroy such as are constructed upon arches, several sorts of barges or boats might be used, which should be filled with fireworks, bombs, petards, &c. It would likewise be extremely easy to construct these machines upon floating rafters, carrying several thousand pounds weight of gunpowder, which might be confined within strong pieces of wood, put together in the manner already described.

These machines should be piled one above the other, and long iron bars must be thrown across the floats, or be fixed like masts, so that when the whole of the combustible materials is beneath the centre of the bridge, the rafters may be stopped. Great care must be taken to dispose the matches in such a manner that no fire may be communicated to the gunpowder before the machine reaches the exact spot which is to be destroyed.

MACHINE, in general, whatever hath force sufficient to raise or stop the motion of a heavy body.

MACHINES are either simple or compound: the simple ones are the seven mechanical powers, viz. lever, balance, pully, axis, and wheel, screw, and inclined plane. See MECHANICAL POWERS.

If the given power is not able to overcome the given resistance when directly applied, that is, when the power applied is less than the weight or resonance given; then the thing is to be performed by the help of a machine, made with levers, wheels, pullies, screws, &c. so adjusted, that when the weight and power are put in motion on the machine, the velocity of the power may be at least so much greater than that of the weight, as the weight and friction of the machine, taken together, is greater than the power; for on this principle depends the mechanism or contrivance of all mechanical engines used to draw or raise heavy bodies, or overcome any other force; the whole design of these being to give such a velocity to the power, in respect of the weight, as that the momentum of the power may exceed the momentum of the weight: for if machines are so contrived, that the velocity of the agent and resistant are reciprocally as their forces, the agent will just sustain the resistant, but with a greater degree of velocity will overcome it. So that if the excess of motion or velocity in the power is so great as to overcome all that resistance which commonly arises from the friction or attraction of contiguous bodies, as they slide by one another, or from the cohesion of bodies that are to be separated, or from the weights of bodies that are to be raised: the excess of the force remaining, after all these resistances are overcome, will produce an acceleration of motion thereto, as well in the parts of the machine, as in the resisting body.

Compound MACHINES, are formed by various combinations, and serve for different purposes; in all which the same general law takes place, viz. that the power and weight sustain each other, when they are in the inverse proportion of the velocities they would have in the directions wherein they act, if they were put in motion. Now, to apply this law to any compound machine, there are four things to be considered: 1. The moving power, or the force that puts the machine in motion; which may be either men or other animals, weights, springs, the wind, a stream of water, &c. 2. The velocity of this power, or the space it moves over in a given time. 3. The resistance, or quantity of weight to be removed. 4. The velocity of this weight, or the space it moves over in the same given time.

The two first of these quantities are always in the reciprocal proportion of the two last; that is, the product of the first two must always be equal to that of the last; hence, three of these quantities being given, it is easy to find the fourth; for example, if the quantity of the power be 4, its velocity 15, and the velocity of the weight 2, then the resistance, or quantity of the weight, will be equal to

4 × 15 60 ------ = -- = 30. 2 2

The following rules will direct the mechanic how he may contrive his machine, that it may answer the intended purpose, to the best advantage.

1. Having assigned the proportion of your power, and the weight to be raised, the next thing is to consider how to combine levers, wheels, pullies, &c. so that working together they may be able to give a velocity to the power, which shall be to that of the weight something greater than in the proportion of the weight to the power. This done, you must estimate your quantity of friction; and if the velocity of the power be to that of the weight still in a greater proportion than the weight and friction taken together are to the power; then your machine will be able to raise the weight. And note, this proportion must be so much greater, as you would have your engine work faster.

2. But the proportion of the velocity of the power and weight must not be made too great: for it is a fault to give a machine too much power, as well as too little; for if the power can raise the weight and overcome the resistance, and the engine perform its proper effect in a convenient time and work well, it is sufficient for the end proposed; and it is in vain to make additions to the engine to increase the power any farther; for that would not only be a needless expence, but the engine would lose time in working.

3. As to the power applied to work the engine, it may either be a living power, as men, horses, &c. or an artificial power, as a spring, &c. or a natural power, as wind, water, fire, weights, &c.

When the quantity of the power is known, it matters not, as to the effect, what kind of power it is; for the same quantity of any sort will produce the same effect; and different sorts of powers may be applied in an equal quantity a great variety of ways.

The most easy power applied to a machine is weight, if it be capable of effecting the thing designed. If not, then wind, water, &c. if that can be conveniently had, and without much expence.

A spring is also a convenient moving power for several machines: but it never acts equally as the weight does; but is stronger when much bent, than when but a little bent, and that in proportion to the bending, or the distance it is forced to; but springs grow weaker by often bending or remaining long bent: yet they recover part of their strength by lying unbent.

The natural powers, wind and water, may be applied to vast advantage in working great engines, when managed with skill and judgment.--The due application of these has much abridged the labors of men; for there is scarce any labor to be performed, but an ingenious artificer can tell how to apply these powers to execute his design, and answer his purpose; for any constant motion being given, it may, by due application, be made to produce any other motions we desire. Therefore these powers are the most easy and useful, and of the greatest benefit to mankind. Besides, they cost nothing, and do not require any repetition nor renewing, like a weight or a spring, which require to be wound up. When these cannot be had, or cannot serve our end, we have recourse to some living power, as men, horses, &c.

4. Men may apply their strength several ways in working a machine. A man of ordinary strength, turning a roller by the handle, can act for a whole day against a resistance equal to 30 pounds weight; and if he works ten hours in a day, he will raise a weight 30lb. 3¹⁄₂ feet in a second; or if the weight be greater, he will raise it so much less in proportion.

But a man may act, for a small time, against a resistance of 50lb. or more.

If two men work at a windlass or roller, they can more easily draw up 70lb. than one man 30lb. provided the elbow of one of the handles be at right angles to that of the other: and with a fly or heavy wheel applied to it, a man may do ¹⁄₃d part more work; and for a little while act with a force, or overcome a continual resistance of 80lb. and work a whole day when the resistance is but 40lb.

Men used to carrying weighty burdens, such as porters, will carry some 150lb. others 200lb. or 250lb. according to their strength.

A man can draw but about 70 or 80lb. horizontally; for he can but apply half his weight.

If the weight of a man be 140lb. he can act with no greater force in thrusting horizontally, at the height of his shoulders, than 27lb.

A horse draws to greatest advantage, when the line of direction is a little elevated above the horizon, and the power acts against his breast: and can draw 200lb. for eight hours in a day, at two miles and an half an hour. If he draws 240lb. he can work but six hours, and not quite so fast; and, in both cases, if he carries some weight he will draw better than if he carried none. And this is the weight a horse is supposed to be able to draw over a pully out of a well. In a cart a horse may draw 1000lb. The most force a horse can exert is when he draws something above a horizontal direction.

The worst way of applying the strength of a horse, is to make him draw or carry up a hill: and three men with 100lb. on their backs, will climb up a steep hill faster than a horse with 300lb.

A round walk for a horse to draw in at a mill, &c. should not be less than 40 feet diameter.

5. Every machine should be made of as few parts, and those as simple as possible, to answer its purpose; not only because the expence of making and repairing will be less, but it will also be less liable to be put out of order.