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 bootlaces: 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 catgut 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 rewind

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

hundredweight, 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 flatbottomed 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 forepart 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 forepart.



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, and without this many of our

mines could not be worked velocity, 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.



NOTES:



1. See Transactions of the Society of Arts, 1826.





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