On the Economy of Machinery and Manufactures/Chapter 7
(56.) It requires some skill and a considerable apparatus to enable many men to exert their whole force at a given point; and when this number amounts to hundreds or to thousands, additional difficulties present themselves. If ten thousand men were hired to act simultaneously, it would be exceedingly difficult to discover whether each exerted his whole force, and consequently, to be assured that each man did the duty for which he was paid. And if still larger bodies of men or animals were necessary, not only would the difficulty of directing them become greater, but the expense would increase from the necessity of transporting food for their subsistence.
The difficulty of enabling a large number of men to exert their force at the same instant of time has been almost obviated by the use of sound. The whistle of the boatswain performs this service on board ships; and in removing, by manual force, the vast mass of granite, weighing above 1400 tons, on which the equestrian figure of Peter the Great is placed at St. Petersburgh, a drummer was always stationed on its summit to give the signal for the united efforts of the workmen.
An ancient Egyptian drawing was discovered a few years since, by Champollion, in which a multitude of men appeared harnessed to a huge block of stone, on the top of which stood a single individual with his hands raised above his head, apparently in the act of clapping them, for the purpose of insuring the exertion of their combined force at the same moment of time.
(57.) In mines, it is sometimes necessary to raise or lower great weights by capstans requiring the force of more than one hundred men. These work upon the surface; but the directions must be communicated from below, perhaps from the depth of two hundred fathoms. This communication, however, is accomplished with ease and certainty by signals: the usual apparatus is a kind of clapper placed on the surface close to the capstan, so that every man may hear, and put in motion from below by a rope passing up the shaft.
At Wheal Friendship mine in Cornwall, a different contrivance is employed: there is in that mine an inclined plane, passing underground about two-thirds of a mile in length. Signals are communicated by a continuous rod of metal, which being struck below, the blow is distinctly heard on the surface.
(58.) In all our larger manufactories numerous instances occur of the application of the power of steam to overcome resistances which it would require far greater expense to surmount by means of animal labour. The twisting of the largest cables, the rolling, hammering, and cutting large masses of iron, the draining of our mines, all require enormous exertions of physical force continued for considerable periods of time. Other means are had recourse to when the force required is great, and the space through which it is to act is small. The hydraulic press of Bramah can, by the exertion of one man, produce a pressure of 1,500 atmospheres; and with such an instrument a hollow cylinder of wrought iron three inches thick has been burst. In rivetting together the iron plates, out of which steam-engine boilers are made, it is necessary to produce as close a joint as possible. This is accomplished by using the rivets red-hot: while they are in that state the two plates of iron are rivetted together, and the contraction which the rivet undergoes in cooling draws them together with a force which is only limited by the tenacity of the metal of which the rivet itself is made.
(59.) It is not alone in the greater operations of the engineer or the manufacturer, that those vast powers which man has called into action, in availing himself of the agency of steam, are fully developed. Wherever the individual operation demanding little force for its own performance is to be multiplied in almost endless repetition, commensurate power is required. It is the same "giant arm" which twists "the largest cable," that spins from the cotton plant an "almost gossamer thread." Obedient to the hand which called into action its resistless powers, it contends with the ocean and the storm, and rides triumphant through dangers and difficulties unattempted by the older modes of navigation. It is the same engine that, in its more regulated action, weaves the canvass it may one day supersede; or, with almost fairy fingers, entwines the meshes of the most delicate fabric that adorns the female form.[1]
(60.) The Fifth Report of the Select Committee of the House of Commons on the Holyhead Roads furnishes ample proof of the great superiority of steam vessels. The following extracts are taken from the evidence of Captain Rogers, the commander of one of the packets:
"Quest. Are you not perfectly satisfied, from the experience you have had, that the steam vessel you command is capable of performing what no sailing vessel can do?"
"Ans. Yes."
"Quest. During your passage from Gravesend to the Downs, could any square-rigged vessel, from a first-rate down to a sloop of war, have performed the voyage you did in the time you did it in the steam boat?"
"Ans. No; it was impossible. In the Downs we passed several Indiamen, and 150 sail there that could not move down the channel; and at the back of Dungeness we passed 120 more."
"Quest. At the time you performed that voyage, with the weather you have described, from the Downs to Milford, if that weather had continued twelve months, would any square-rigged vessel have performed it?"
"Ans. They would have been a long time about it; probably, would have been weeks instead of days. A sailing vessel would not have beat up to Milford, as we did, in twelve months."
(61.) The process of printing on the silver paper, which is necessary for bank-notes, is attended with some inconvenience, from the necessity of damping the paper previously to taking the impression. It was difficult to do this uniformly; and in the old process of dipping a parcel of several sheets together into a vessel of water, the outside sheets becoming much more wet than the others, were very apt to be torn. A method has been adopted at the Bank of Ireland which obviates this inconvenience. The whole quantity of paper to be damped is placed in a close vessel from which the air is exhausted; water is then admitted, and every leaf is completely wetted; the paper is then removed to a press, and all the superfluous moisture is squeezed out.
(62.) The operation of pulverizing solid substances and of separating the powders of various degrees of fineness, is common in the arts: and as the best graduated sifting fails in effecting this separation with sufficient delicacy, recourse is had to suspension in a fluid medium. The substance when reduced by grinding to the finest powder is agitated in water, which is then drawn off: the coarsest portion of the suspended matter first subsides, and that which requires the longest time to fall down is the finest. In this manner even emery powder, a substance of great density, is separated into the various degrees of fineness which are required. Flints, after being burned and ground, are suspended in water, in order to mix them intimately with clay, which is also suspended in the same fluid, for the formation of porcelain. The water is then in part evaporated by heat, and the plastic compound, out of which our most beautiful porcelain is formed, remains. It is a curious fact, and one which requires further examination than it has yet received, that, if this mixture be suffered to remain long at rest before it is worked up, it becomes useless; for it is then found that the silex, which at first was uniformly mixed, becomes aggregated together in small lumps. This parallel to the formation of flints in the chalk strata deserves attention.[2]
(63.) The slowness with which powders subside, depends partly on the specific gravity of the substance, and partly on the magnitude of the particles themselves. Bodies, in falling through a resisting medium, after a certain time acquire a uniform velocity, which is called their terminal velocity, with which they continue to descend: when the particles are very small, and the medium dense, as water, this terminal velocity is soon arrived at. Some of the finer powders even of emery require several hours to subside through a few feet of water, and the mud pumped up into our cisterns by some of the water companies is suspended during a still longer time. These facts furnish us with some idea of the great extent over which deposits of river mud may be spread; for if the mud of any river whose waters enter the Gulph stream, sink through one foot in an hour, it might be carried by that stream 1500 miles before it had sunk to the depth of 600 or 700 feet.
(64.) A number of small filaments of cotton ⟨project⟩ from even the best spun thread, and when this thread is woven into muslin they injure its appearance. To cut these off separately is quite impossible, but they are easily removed by passing the muslin rapidly over a cylinder of iron kept at a dull red heat: the time during which each portion of the muslin is in contact with the red-hot iron is too short to heat it to the burning point; but the filaments being much finer, and being pressed close to the hot metal, are burnt.
The removal of these filaments from patent net is still more necessary for its perfection. The net is passed at a moderate velocity through a flame of gas issuing from a very long and narrow slit. Immediately above the flame a long funnel is fixed, which is connected with a large air pump worked by a steam engine. The flame is thus drawn forcibly through the net, and all the filaments on both sides of it are burned off at one operation. Previously to this application of the air pump, the net acting in the same way, although not to the same extent, as the wire-gauze in Davy's safety lamp, cooled down the flame so as to prevent the combustion of the filaments on the upper side: the air pump by quickening the current of ignited gas, removes this inconvenience.
- ↑ The importance and diversified applications of the steam-engine were most ably enforced in the speeches made at a public meeting, held (June 1824) for the purpose of proposing the erection of a monument to the memory of James Watt; these were subsequently printed.
- ↑ Some observations on this subject, by Dr. Fitton, occur in the appendix to Captain King's Survey of the Coast of Australia, vol. ii. p. 397. London, 1826.