CHAPTER 4.

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.