On the Economy of Machinery and Manufactures/Chapter 4
CHAP. IV.
INCREASE AND DIMINUTION OF VELOCITY.
(32.) The fatigue produced on the muscles of the human frame does not altogether depend on the actual force employed in each effort, but partly on the frequency with which it is exerted. The exertion necessary to accomplish every operation consists of two parts: one of these is the expenditure of force which is necessary to drive the tool or instrument; and the other is the effort required for the motion of some limb of the animal producing the action. In driving a nail into a piece of wood, one of these is lifting the hammer, and propelling its head against the nail; the other is, raising the arm itself, and moving it in order to use the hammer. If the weight of the hammer is considerable, the former part will cause the greatest portion of the exertion. If the hammer is light, the exertion of raising the arm will produce the greatest part of the fatigue. It does therefore happen, that operations requiring very trifling force, if frequently repeated, will tire more effectually than more laborious work. There is also a degree of rapidity beyond which the action of the muscles cannot be pressed.
(33.) The most advantageous load for a porter who carries wood up stairs on his shoulders, has been investigated by M. Coulomb; but he found from experiment that a man walking up stairs without any load, and raising his burden by means of his own weight in descending, could do as much work in one day, as four men employed in the ordinary way with the most favourable load.
(34.) The proportion between the velocity with which men or animals move, and the weights they carry, is a matter of considerable importance, particularly in military affairs. It is also of great importance for the economy of labour, to adjust the weight of that part of the animal's body which is moved, the weight of the tool it urges, and the frequency of repetition of these efforts, so as to produce the greatest effect. An instance of the saving of time by making the same motion of the arm execute two operations instead of one, occurs in the simple art of making the tags of boot-laces: these tags are formed out of very thin, tinned, sheet-iron, and were formerly cut out of long strips of that material into pieces of such a breadth that when bent round they just enclosed the lace. Two pieces of steel have recently been fixed to the side of the shears, by which each piece of tinned-iron as soon as it is cut is bent into a semi-cylindrical form. The additional power required for this operation is almost imperceptible; and it is executed by the same motion of the arm which produces the cut. The work is usually performed by women and children; and with the improved tool more than three times the quantity of tags is produced in a given time.[1]
(35.) Whenever the work is itself light, it becomes necessary, in order to economize time, to increase the velocity. Twisting the fibres of wool by the fingers would be a most tedious operation: in the common spinning-wheel the velocity of the foot is moderate, but by a very simple contrivance that of the thread is most rapid. A piece of cat-gut passing round a large wheel, and then round a small spindle, effects this change. This contrivance is common to a multitude of machines, some of them very simple. In large shops for the retail of ribands, it is necessary at short intervals to "take stock," that is, to measure and re-wind every piece of riband, an operation which, even with this mode of shortening it, is sufficiently tiresome, but without it would be almost impossible from its expense. The small balls of sewing-cotton, so cheap and so beautifully wound, are formed by a machine on the same principle, and but a few steps more complicated.
(36.) In turning from the smaller instruments in frequent use to the larger and more important machines, the economy arising from the increase of velocity becomes more striking. In converting cast into wrought iron, a mass of metal, of about a hundred weight, is heated almost to white heat, and placed under a heavy hammer moved by water or steam power. This is raised by a projection on a revolving axis; and if the hammer derived its momentum only from the space through which it fell, it would require a considerably greater time to give a blow. But as it is important that the softened mass of red-hot iron should receive as many blows as possible before it cools, the form of the cam or projection on the axis is such, that the hammer, instead of being lifted to a small height, is thrown up with a jerk, and almost the instant after it strikes against a large beam, which acts as a powerful spring, and drives it down on the iron with such velocity that by these means about double the number of strokes can be made in a given time. In the smaller tilt-hammers, this is carried still further: by striking the tail of the tilt-hammer forcibly against a small steel anvil, it rebounds with such velocity, that from three to five hundred strokes are made in a minute. In the manufacture of anchors, an art in which a similar contrivance is of still greater importance, it has only been recently applied.
(37.) In the manufacture of scythes, the length of the blade renders it necessary that the workman should move readily, so as to bring every part of it on the anvil in quick succession. This is effected by placing him in a seat suspended by ropes from the ceiling: so that he is enabled, with little bodily exertion, to vary his distance, by pressing his feet against the block which supports the anvil, or against the floor.
(38.) An increase of velocity is sometimes necessary to render operations possible: thus a person may skate with great rapidity over ice which would not support his weight if he moved over it more slowly. This arises from the fact, that time is requisite for producing the fracture of the ice: as soon as the weight of the skater begins to act on any point, the ice, supported by the water, bends slowly under him; but if the skater's velocity is considerable, he has passed off from the spot which was loaded before the bending has reached the point which would cause the ice to break.
(39.) An effect not very different from this might take place if very great velocity were communicated to boats. Let us suppose a flat-bottomed boat, whose bow forms an inclined plane with the bottom, at rest in still-water. If we imagine some very great force suddenly to propel this boat, the inclination of the plane at the fore-part would cause it to rise in the water; and if the force were excessive, it might even rise out of the water, and advance, by a series of leaps, like a piece of slate or an oyster shell, thrown as a "duck and drake."
If the force were not sufficient to pull the boat out of the water, but were just enough to bring its bottom to the surface, it would be carried along with a kind of gliding motion with great rapidity; for at every point of its course it would require a certain time before it could sink to its usual draft of water; but before that time had elapsed, it would have advanced to another point, and consequently have been raised by the reaction of the water on the inclined plane at its fore-part.
(40.) The same fact, that bodies moving with great velocity have not time to exert the full effect of their weight, seems to explain a circumstance which appears to be very unaccountable. It sometimes happens that when foot-passengers are knocked down by carriages, the wheels pass over them with scarcely any injury, though, if the weight of the carriage had rested on their body, even for a few seconds, it would have crushed them to death. If the view above taken is correct, the injury in such circumstances will chiefly happen to that part of the body which is struck by the advancing wheel.
(41.) An operation in which rapidity is of essential importance is in bringing the produce of mines up to the surface. The shafts through which the produce is raised are sunk at a very great expense, and it is, of course, desirable to sink as few of them as possible. The matter to be extracted is therefore raised by steam-engines with considerable velocity; and without this many of our mines could not be worked with profit.
(42.) The effect of great velocity in modifying the form of a cohesive substance is beautifully shown in the process for making window-glass, termed "flashing," which is one of the most striking operations in our domestic arts. A workman having dipped his iron tube into the glass pot, and loaded it with several pounds of the melted "metal," blows out a large globe, which is connected with his rod by a short thick hollow neck. Another workman now fixes to the globe immediately opposite to its neck, an iron rod, the extremity of which has been dipped in the melted glass; and when this is firmly attached, a few drops of water separate the neck of the globe from the iron tube. The rod with the globe attached to it is now held at the mouth of a glowing furnace: and by turning the rod the globe is made to revolve slowly, so as to be uniformly exposed to the heat: the first effect of this softening is to make the glass contract upon itself and to enlarge the opening of the neck. As the softening proceeds, the globe is turned more quickly on its axis, and when very soft and almost incandescent, it is removed from the fire, and the velocity of rotation being still continually increased, the opening enlarges from the effect of the centrifugal force, at first gradually, until at last the mouth suddenly expands or "flashes" out into one large circular sheet of red-hot glass. The neck of the original globe, which is to become the outer part of the sheet, is left thick to admit of this expansion, and forms the edge of the circular plate of glass, which is called a "Table." The centre presents the appearance of a thick boss or prominence, called the "Bull's-eye," at the part by which it was attached to the iron rod.
(43.) The most frequent reason for employing contrivances for diminishing velocity, arises from the necessity of overcoming great resistances with small power. Systems of pulleys, the crane, and many other illustrations, might also be adduced here as examples; but they belong more appropriately to some of the other causes which we have assigned for the advantages of machinery. The common smoke-jack is an instrument in which the velocity communicated is too great for the purpose required, and it is transmitted through wheels which reduce it to a more moderate rate.
(44.) Telegraphs are machines for conveying information over extensive lines with great rapidity. They have generally been established for the purposes of transmitting information during war, but the increasing wants of man will probably soon render them subservient to more peaceful objects.
A few years since the telegraph conveyed to Paris information of the discovery of a comet, by M. Gambart, at Marseilles: the message arrived during a sitting of the French Board of Longitude, and was sent in a note from the Minister of the Interior to Laplace, the President, who received it whilst the writer of these lines was sitting by his side. The object in this instance was, to give the earliest publicity to the fact, and to assure to M. Gambart the title of its first discoverer.
At Liverpool a system of signals is established for the purposes of commerce, so that each merchant can communicate with his own vessel long before she arrives in the port.
- ↑ See Transactions of the Society of Arts, 1826.