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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.

Chapter 5

Extending the Time of Action of Forces

 

45. This is one of the most common and most useful of the

employments of machinery. The half minute which we daily devote

to the winding-up of our watches is an exertion of labour almost

insensible; yet, by the aid of a few wheels, its effect is spread

over the whole twenty-four hours. In our clocks, this extension

of the time of action of the original force impressed is carried

still further; the better kind usually require winding up once in

eight days, and some are occasionally made to continue in action

during a month, or even a year. Another familiar illustration may

be noticed in our domestic furniture: the common jack by which

our meat is roasted, is a contrivance to enable the cook in a few

minutes to exert a force which the machine retails out during the

succeeding hour in turning the loaded spit; thus enabling her to

bestow her undivided attention on the other important duties of

her vocation. A great number of automatons and mechanical toys

moved by springs, may be classed under this division.

 

46. A small moving power, in the shape of a jack or a spring

with a train of wheels, is often of great convenience to the

experimental philosopher, and has been used with advantage in

magnetic and electric experiments where the rotation of a disk of

metal or other body is necessary, thus allowing to the enquirer

the unimpeded use of both his hands. A vane connected by a train

of wheels, and set in motion by a heavy weight, has also, on some

occasions, been employed in chemical processes, to keep a

solution in a state of agitation. Another object to which a

similar apparatus may be applied, is the polishing of small

specimens of minerals for optical experiments.

Chapter 6

Saving time in Natural Operations

 

47. The process of tanning will furnish us with a striking

illustration of the power of machinery in accelerating certain

processes in which natural operations have a principal effect.

The object of this art is to combine a certain principle called

tanning with every particle of the skin to be tanned. This, in

the ordinary process, is accomplished by allowing the skins to

soak in pits containing a solution of tanning matter: they remain

in the pits six, twelve, or eighteen months; and in some

instances (if the hides are very thick), they are exposed to the

operation for two years, or even during a longer period. This

length of time is apparently required in order to allow the

tanning matter to penetrate into the interior of a thick hide.

The improved process consists in placing the hides with the

solution of tan in close vessels, and then exhausting the air.

The effect is to withdraw any air which may be contained in the

pores of the hides, and to aid capillary attraction by the

pressure of the atmosphere in forcing the tan into the interior

of the skins. The effect of the additional force thus brought

into action can be equal only to one atmosphere, but a further

improvement has been made: the vessel containing the hides is,

after exhaustion, filled up with a solution of tan; a small

additional quantity is then injected with a forcing-pump. By

these means any degree of pressure may be given which the

containing vessel is capable of supporting; and it has been found

that, by employing such a method, the thickest hides may be

tanned in six weeks or two months.

 

48. The same process of injection might be applied to

impregnate timber with tar, or any other substance capable of

preserving it from decay, and if it were not too expensive, the

deal floors of houses might thus be impregnated with alumine or

other substances, which would render them much less liable to be

accidentally set on fire. In some cases it might be useful to

impregnate woods with resins, varnish, or oil; and wood saturated

with oil might, in some instances, be usefully employed in

machinery for giving a constant, but very minute supply of that

fluid to iron or steel, against which it is worked. Some idea of

the quantity of matter which can be injected into wood by great

pressure, may be formed, from considering the fact stated by Mr

Scoresby, respecting an accident which occurred to a boat of one

of our whaling-ships. The harpoon having been struck into the

fish, the whale in this instance, dived directly down, and

carried the boat along with him. On returning to the surface the

animal was killed, but the boat, instead of rising, was found

suspended beneath the whale by the rope of the harpoon; and on

drawing it up, every part of the wood was found to be so

completely saturated with water as to sink immediately to the

bottom.

 

49. The operation of bleaching linen in the open air is one

for which considerable time is necessary; and although it does

not require much labour, yet, from the risk of damage and of

robbery from long /exposure, a mode of shortening the process was

highly desirable. The method now practised, although not

mechanical, is such a remarkable instance of the application of

science to the practical purposes of manufactures, that in

mentioning the advantages derived from shortening natural

operations, it would have been scarcely pardonable to have

omitted all allusion to the beautiful application of chlorine, in

combination with lime, to the art of bleaching.

 

50. Another instance more strictly mechanical occurs in some

countries where fuel is expensive, and the heat of the sun is not

sufficient to evaporate the water from brine springs. The water

is first pumped up to a reservoir, and then allowed to fall in

small streams through faggots. Thus it becomes divided; and,

presenting a large surface, evaporation is facilitated, and the.

brine which is collected in the vessels below the faggots is

stronger than that which was pumped up. After thus getting rid of

a large part of the water, the remaining portion is driven off by

boiling. The success of this process depends on the condition of

the atmosphere with respect to moisture. If the air, at the time

the brine falls through the faggots, holds in solution as much

moisture as it can contain in an invisible state, no more can be

absorbed from the salt water, and the labour expended in pumping

is entirely wasted. The state of the air, as to dryness, is

therefore an important consideration in fixing the time when this

operation is to be performed; and an attentive examination of its

state, by means of the hygrometer, might be productive of some

economy of labour.

 

51. In some countries, where wood is scarce, the evaporation

of salt water is carried on by a large collection of ropes which

are stretched perpendicularly. In passing down the ropes, the

water deposits the sulphate of lime which it held in solution,

and gradually incrusts them, so that in the course of twenty

years, when they are nearly rotten, they are still sustained by

the surrounding incrustation, thus presenting the appearance of a

vast collection of small columns.

 

52. Amongst natural operations perpetually altering the

surface of our globe, there are some which it would be

advantageous to accelerate. The wearing down of the rocks which

impede the rapids of navigable rivers, is one of this class. A

very beautiful process for accomplishing this object has been

employed in America. A boat is placed at the bottom of the rapid,

and kept in its position by a long rope which is firmly fixed on

the bank of the river near the top. An axis, having a wheel

similar to the paddle-wheel of a steamboat fixed at each end of

it, is placed across the boat; so that the two wheels and their

connecting axis shall revolve rapidly, being driven by the force

of the passing current. Let us now imagine several beams of wood

shod with pointed iron fixed at the ends of strong levers,

projecting beyond the bow of the boat, as in the annexed

representation.

 

If these levers are at liberty to move up and down, and if

one or more projecting pieces, called cams, are fixed on the axis

opposite to the end of each lever, the action of the stream upon

the wheels will keep up a perpetual succession of blows. The

sharp-pointed shoe striking upon the rock at the bottom, will

continually detach small pieces, which the stream will

immediately carry off. Thus, by the mere action of the river

itself, a constant and most effectual system of pounding the rock

at its bottom is established. A single workman may, by the aid of

a rudder, direct the boat to any required part of the stream; and

when it is necessary to move up the rapid, as the channel is cut,

he can easily cause the boat to advance by means of a capstan.

 

53. When the object of the machinery just described has been

accomplished, and the channel is sufficiently deep, a slight

alteration converts the apparatus to another purpose almost

equally advantageous. The stampers and the projecting pieces on

the axis are removed, and a barrel of wood or metal, surrounding

part of the axis, and capable, at pleasure, of being connected

with, or disconnected from the axis itself, is substituted. The

rope which hitherto fastened

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