The first use of flint in pottery has been thus explained. A potter named Astbury, travelling to London, perceived something amiss with one of his horse’s eyes, when an ostler at Dunstable said he could cure him, and for that purpose put a common black flint into the fire. The potter observing it when taken out to be of a fine white, immediately conceived the idea of improving his ware by the addition of this material to the clay.
Imposing Mechanical Effects.
Mechanical force, when exerted even as a motive-power, can be employed by man on many a grand scale. The movements of massive pieces of machinery, even though moving aimlessly, still more when working for a purpose, always awaken in us the idea of power; and often also create emotions of awe and sublimity akin to those which are begotten by the spectacle of great natural phenomena. The sweep of a railway train across the country, and the dash of a war-steamer against the waves with which it measures its strength, never become paltry pageants, even though we are ignorant of the errands on which these swift coursers are bound. Still more striking are those actions of machinery which involve not only swift irresistible motion, but also transformation of the materials on which the moving force is exerted. Take, for example, a cotton-mill, which some never tire of representing as dreary and prosaic. In the basement story revolves an immense steam-engine, unresting and unhasting as a star, in its stately, orderly movements. It stretches its strong iron arms in every direction throughout the building; and into whatever chamber you enter, as you climb stair after stair, you find its million hands in motion, and its fingers, which are as skilful as they are nimble, busy at work. They pick cotton and cleanse it, card it, rove it, twist it, spin it, dye it, and weave it. They will work any pattern you select, and in as many colours as you choose; and do all with such celerity, dexterity, unexhausted energy, and skill, that you begin to see what was prefigured in the legend of Michael Scott, and his “sabbathless” demons (as Charles Lamb would have called them), to whom the most hateful of all things was rest, and ropemaking, though it were of sand, more welcome than idleness. For our own part, we gaze with untiring wonder and admiration on the steam Agathodæmons of a cotton-mill, the embodiments, all of them, of a few very simple statical and dynamical laws; and yet able, with the speed of race-horses, to transform a raw material, originally as cheap as thistle-down, into endless useful and beautiful fabrics. Michael Scott, had he lived to see them, would have dismissed his demons and broken his wand.—Prof. George Wilson.[15]
Horse-power.
In speaking of the power, or force which an engine exerts, it is necessary to have some measure of force, or standard of inference. That used in this country is a Horse-power, a force equal to that which the average strength of a horse was believed capable of exerting. This has been estimated at 33,000 lb. avoirdupois weight, raised one foot high in a minute. There have been different estimates as to the real power of horses; and it is now considered that taking the most advantageous rate, for using horse-power, the medium power of that animal is equal to 22,000 lb. raised one foot high per minute. However, the other 33,000 lb. is taken as the standard, and is what is meant when a horse-power is spoken of. In comparing the power of a steam-engine with that of horses applied to do the same work, it must be remembered that the engine horse-power is 33,000 lb. raised one foot per minute; the real horse-power only 22,000 lb.; and that the engine will work unceasingly for twenty-four hours, while the horse works at that rate only eight hours. The engine works three times as long as the horse; hence, to do the same work in a day as the engine of one horse-power, 4·5 horses would be required (33,000 × 3 = 99,000; 99,000 ÷ 22,000 = 4·5). The power of a man may be estimated at one-fifth of the real power of a horse, or 44,000 lb. raised one foot per minute.—Hugo Reid on the Steam-Engine.
The First Practical Steam-boat.
Mr. Macquorn Rankine, in supporting the opinion of Mr. Benet Woodcroft, that the title of the “first practical steamboat” is due to that vessel in which the double-acting cranked steam-engine—in short, Watt’s rotative engine—was first applied to drive the propeller,—proceeds on the principle, that to constitute a “practical” machine, that machine must be capable, not merely of working well during a series of experiments, but of continuing to work well for years, with ordinary care in its management and repairs. Such certainly never was, and never could have been the case, with any steam-boat in which the wheels were made to turn by means of chains and rachet-work—a sort of mechanism which may answer its purpose during an experiment, but which must rapidly wear itself out by shocks and rattling. Such an engine is not a “practical steam-engine;” and a vessel driven by it is not a “practical steam-boat.” Hence the importance which Mr. Rankine is disposed to ascribe to the first actual use of a permanently efficient rotative steam-engine to drive a vessel.
It may be true that as an original inventor, Symington ought to be ranked below his predecessors; because his steam-boat of 1801 was only a new combination of parts which had previously been invented separately by others—the paddle-wheel, by some unknown mechanic of remote antiquity; the application of steam to drive vessels, by a series of inventors, comprising Papin, Hulls, D. Bernouilli, Jouffroy, Miller, and Taylor; and the rotative steam-engine by Watt: still, the merit of having first used a “practical steam-engine” to drive a vessel is due to Symington.—Communicated to the Literary and Philosophical Society of Manchester, 1863.
Effect of Heavy Seas upon Large Vessels.
Professor Tennant, in considering the effect of heavy seas upon vessels of 400 to 600 feet long, remarks that the waves of the Atlantic are stated, by some captains of American “liners,” to attain an elevation of 20 feet, with a length of 160 feet, and a velocity of 25 to 30 miles per hour. Dr. Scoresby, in his paper on Atlantic waves, gives about the same mean elevation for the waves in rather a hard gale a-head; on one occasion, with a hard gale and heavy squalls, some few waves attained a height of 43 feet, with a length of nearly 600 feet, and a velocity exceeding 30 miles an hour. Other authorities assume even more than those heights and distances. The amount of strength, to resist the impact of such waves, must vary with the length and size of a ship, and the materials of which it was constructed; and as the experience of the Britannia Bridge shows, that a weight of 460 tons, at a velocity of 30 miles per hour, could be borne by a cellular tube of 460 feet span, it was demonstrated, that by the use of iron, almost any amount of strength could be given to a vessel; and as stability could be imparted by proper proportions, efficient vessels could be built of any dimensions, as has been exemplified by the Great Britain, which after remaining ashore on rocks for several months, had been got off without serious injury.