Читать книгу Continental Monthly , Vol. 6, No. 1, July, 1864 - Various - Страница 5

And, first, of electricity. This has been cultivated with the greatest success in our country, from the time when Franklin with his kite drew down electricity from the thunder cloud, to that when Henry showed the electrical currents produced by the distant lightning discharge. In Franklin's day the idea prevailed that there were two kinds of electricity, one produced by rubbing vitreous substances, the other by the friction of resinous bodies. Franklin's theory of one electric fluid in all bodies, disturbed in its equilibrium by friction, and thus accumulating in one and deserting the other, maintains its ground, still capable of explaining the facts elicited in the progress of modern discovery. Franklin believed that electricity and lightning were the same, and proceeded to the proof. He made the perilous experiment, by exploring the air with a kite, and drawing down from the thunder cloud the lightning's discharge upon his own person. The bold philosopher received unharmed the shock of the electric fluid, more fortunate than others who have fallen victims to less daring experiments. The world was delighted with the discoveries of the great American, and for a time electricity was called Franklinism on the continent of Europe; but Franklin was born here, and the name was not adopted in England. While Franklin made experiments, Kinnersley exhibited and illustrated them, and also rediscovered the seemingly opposite electricities of glass and resin. Franklin's lightning rod is gradually surmounting the many difficulties with which it contended, as experience attests the greater safety of houses protected by the rod, properly mounted, whilst the British attempt to substitute balls for points has failed. This question, as to powder magazines, has lately excited much controversy. Should a rod be attached to the magazine, or should it be placed upon a post at some distance? This question has been solved by Henry. When an electrical discharge passes from one body to another, the electricity in all the bodies in the neighborhood is affected. Henry magnetized a needle in a long conductor, by the discharge from a cloud, more than a mile from the conductor. If a discharge passes down a rod, attached to a powder house, may it not cause a spark to pass from one receptacle for powder to another, and thus inflame the whole? The electrical plenum, which Henry supposed, is no doubt disturbed, and to great distances; but the effect diminishes with the distance. If all the principal conductors about a building can be connected with a lightning rod, there is no danger of a discharge; for it is only in leaving or entering a conductor that electricity produces heating effects; but if not, the rod is safer at a moderate distance from the building. The rate at which electricity moved was another of the experiments of Franklin. A wire was led over a great extent of ground, and a discharge passed through it. No interval could be perceived between the time of the spark passing to and from the wire at the two ends. Not long since, Wheatston of England, aided by our own great mechanic, Saxton, solved the problem. This has induced Arago, of France, to propose to test the rival theories of light, by similar means—to measure thus a velocity, to detect which has heretofore required a motion over the line of the diameter of the earth's orbit.

In galvanism, our countrymen have made many important discoveries. Dr. Hare invented instruments of such great power as well to deserve the names of calorimeter and deflagrator. The most refractory substances yielded to the action of the deflagrator, melting like wax before a common fire. Even charcoal was supposed to be fused in the experiments of Hare and Silliman, and the visionary speculated on the possibility of black as well as white diamonds. Draper, by his most ingenious galvanic battery, of two metals and two liquids, with one set of elements, in a glass tube not the size of the little finger, was able to decompose water. Faraday, of England, discovered the principle, that when a current of electricity is set in motion, or stopped in a conductor, a neighboring conductor has a current produced in the opposite direction. Henry proved that this principle might be made available to produce an action of a current upon itself, by forming a conductor in the whirls of a spiral, so that sparks and shocks might be obtained by the use of such spirals, when connected with a pair of galvanic plates, a current from which could give no sparks and no shocks. Henry's discoveries of the effects of a current in producing several alternations in currents in neighboring conductors—the change of the quality of electricity which gives shocks to the muscles into that producing heat, and vice versa—his mode of graduating these shocks—his theoretical investigations into the causes of these alternations—are abstruse, but admirable; and his papers have been republished throughout Europe. The heating effects of a galvanic current have been applied by Dr. Hare to blasting. The accidents which so often happen in quarries may be avoided by firing the charge from a distance, as the current which heats the wire, passing through the charge, may be conveyed, without perceptible diminution, through long distances. A feeble attempt to attribute this important invention of Dr. Hare to Colonel Pasley, an English engineer, has been abandoned. This is the marvellous agent by which our eminent countryman, Morse, encouraged by an appropriation made by Congress, will, by means of his electric telegraph, soon communicate information forty miles, from Washington to Baltimore, more rapidly than by whispering in the ear of a friend sitting near us. A telegraph on a new plan at that time, invented by Mr. Grout, of Massachusetts, in 1799, asked a question and received an answer in less than ten minutes through a distance of ninety miles. The telegraph of Mr. Morse will prove, I think, superior to all others; and the day is not distant when, by its aid, we may perhaps ask questions and receive replies across our continent, from ocean to ocean, thus uniting with steam in enlarging the limits over which our Republic may be safely extended.2

Many of our countrymen have contributed to the branch which regards the action of electrified and magnetic bodies. Lukens's application of magnetism to steel (called touching), the compass of Bissel for detecting local attraction, of Burt for determining the variation of the compass, and the observations on the variations of the needle made by Winthrop and Dewitt, deserve notice and commendation. Not long since, Gauss, of Germany, invented instruments by which the changes of magnetic variation and force could be accurately determined. Magnetic action is ever varying. The needle does not point in the same direction for even a few minutes together. The force of magnetism, also, perpetually varies. 'True as the needle to the pole' is not a correct simile for the same place, and, if we pass from one spot to another, is falsified at each change of our position; for the needle changes its direction, and the force varies. Enlarged and united observations, embracing the various portions of the world, must produce important results. The observations at Philadelphia, conducted by Dr. A. D. Bache, and now continued by him under the direction of the Topographical Bureau, are of great value, and will, it is hoped, be published by Congress. Part of them have already first seen the light in Europe—a result much to be regretted, for we are not strong enough in science to spare from the national records the contributions of our countrymen.

These combined observations, progressing throughout the world, are of the highest importance. The University of Cambridge, the American Philosophical Society, and Girard College have erected observatories; and one connected with the Depot of Charts and Instruments has been built in this city last year by the Government, and thoroughly furnished with instruments for complete observations. The names of Bache, Gillis, Pierce, Lovering, and Bond are well known in connection with these establishments.

A magnetic survey of Pennsylvania has been made by private enterprise, and the beginning of a survey in New York. Loomis has observed in Ohio, Locke in Ohio and Iowa, and to him belongs the discovery of the position of the point of greatest magnetic intensity in the Western World. Most interesting magnetic observations (now in progress of publication by Congress) are the result of the toilsome, perilous, and successful expedition, under Commander Wilkes, of our navy, by whom was discovered the Antarctic continent, and a portion of its soil and rock brought home to our country.

The analogy of the auroral displays with those of electricity in motion, was first pointed out by Dr. A. D. Bache, whose researches, in conjunction with Lloyd of Dublin, to determine whether differences of longitude could be measured by the observations of small simultaneous changes in the position of the magnetic needle, led to the knowledge of the curious fact, that these changes, which had been traced as simultaneous, or nearly so, in the continent of Europe, did not so extend across the Atlantic.

Kindred to these two branches are electro-magnetism and magneto-electricity, connected with which, as discoverers, are our countrymen Dana, Green, Hare, Henry, Page, Rogers, and Saxton. The reciprocal machine for producing shocks, invented by Page, and the powerful galvanic magnet of Henry, are entitled to respectful notice. This force, it was thought, might be substituted for steam; but no experiments have as yet established its use, on any important scale, as a motive power. The fact that an electrical spark could be produced by a peculiar arrangement of a coil of wire, connected with a magnet, is a recent discovery; and the first magneto-electric machine capable of keeping up a continuous current was invented by Saxton.

Electricity and magnetism touch in some points upon heat. Heat produces electrical currents; electrical currents produce heat. Heat destroys magnetism. Melted iron is incapable of magnetic influence. Reduction of temperature in iron so far decreases the force, that a celebrated philosopher made an elaborate series of experiments to ascertain whether a great reduction of temperature might not develop magnetic properties in metals other than iron. This branch of thermo-electricity has received from us but little attention. Franklin's experiments, by placing differently colored cloths in the snow, and showing the depth to which they sank, are still quoted, and great praise has been bestowed abroad on a more elaborate series of experiments, by a descendant of his, Dr. A. D. Bache, proving that this law does not hold good as to heat, unaccompanied by light. The experiments of Saxon and Goddard demonstrate that solid bodies do slowly evaporate. It is proper here to mention our countryman, Count Rumford, whose discoveries as to the nature and properties of heat, improvement in stoves and gunnery, and in the structure of chimneys and economy of fuel, have been so great and useful.

Light accompanies heat of a certain temperature. That it acts directly to increase or decrease magnetic force, is not yet proved; and the interesting experiments made by Dr. Draper, in Virginia, go to show that it is without magnetic influence. The discussion of this subject forms, the branch of optics, touching physical science on the one side, the most refined, and the highest range of mathematics on the other. Rittenhouse first suggested the true explanation of the experiment, of the apparent conversion of a cameo into an intaglio, when viewed through a compound microscope, and anticipated many years Brewster's theory. Hopkinson wrote well on the experiment made by looking at a street lamp through a slight texture of silk. Joscelyn, of New York, investigated the causes of the irradiation manifested by luminous bodies, as for instance the stars. Of late, photographic experiments have occupied much attention, and Draper has advanced the bold idea, supported by experiment, that the agent in the so-called photography, is not light, nor heat, but an agent differing from any other known principle. Henry has investigated the luminous emanation from lime, calcined with sulphur, and certain other substances, and finds that it differs much from light in some of its qualities.

Astronomy is the most ancient and highest branch of physics. One of our earliest and greatest efforts in this branch was the invention of the mariner's quadrant, by Godfrey, a glazier of Philadelphia. The transit of Venus, in the last century, called forth the researches of Rittenhouse, Owen, Biddle, and President Smith, near Philadelphia, and of Winthrop, at Boston. Two orreries were made by Rittenhouse, as also a machine for predicting eclipses. Most useful observations, connected with the solar eclipses, from 1832 to 1840, have been made by Paine, of Boston. We have now well-supplied observatories at West Point, Washington, Cambridge, Philadelphia, Hudson, Ohio, and Tuskaloosa, Alabama; and the valuable labors of Loomis, Bartlett, Gillis, Bond, Pierce, Walker, and Kendall are well known. Mr. Adams, so distinguished in this branch and that of weights and measures, laid last year the corner stone of an observatory at Cincinnati, where will soon be one of the largest and most powerful telescopes in the world. Most interesting observations as to the great comet of 1843 were made by Alexander, Anderson, Bartlett, Kendall, Pierce, Walker, Downes, and Loomis, and valuable astronomical instruments have been constructed by Amasa Holcomb, of Massachusetts, and Wm. J. Young, of Philadelphia.

It is difficult to class the brilliant meteors of November the 13th, 1833. If such meteors are periodic, the discovery was made by Professor Olmsted; and Mr. Herrick, of New Haven, has added valuable suggestions. The idea that observers, differently placed at the time of appearance and disappearance of the same meteor, would give the means of determining differences of longitude, was first applied in our own country, where the difference of longitude of Princeton and Philadelphia was determined by observations of Henry and Alexander, Espy and Bache. In meteorology our countrymen have succeeded well. Dr. Wells, of South Carolina, elaborated his beautiful and original theory of the formation of dew, and supported it by many well-devised and conclusive experiments. The series of hourly observations, by Professor Snell and Captain Mordecai, are well known; and the efforts of New York and Pennsylvania, of the medical department of the army, and its present enlightened head, Dr. Lawson, have much advanced this branch of science. The interesting question, Does our climate change? seems to be answered thus far in the negative, by registers kept in Massachusetts and New York. There are two rival theories of storms. That of Redfield, of a rotary motion of a wide column of air, combined with a progressive motion in a curved line. Espy builds on the law of physics, examines the action of an upmoving column of air, shows the causes of its motion and the results, and then deduces his most beautiful theory of rain and of land and water spouts. This he puts to the test of observation; and in the inward motion of wind toward the centre of storms, finds a striking verification of his theory. This theory is also sustained by the overthrow or injury, in the recent tornado at Natchez, of the houses whose doors and windows were closed, while those which were open mostly escaped unhurt. Mr. Espy must be considered, not only here, but throughout the world, as at the head of this branch of science. This subject has been greatly advanced by Professor Loomis, whose paper has been pronounced, by the highest authority, to be the best specimen of inductive reasoning which meteorology has produced. The most recent and highly valuable meteorological works of Dr. Samuel Forry are much esteemed. Many important discoveries in pneumatics were made by Dr. Franklin and Count Rumford, and the air pump was also greatly improved by Dr. Prince, of Salem.

Chemistry, in all its departments, has been successfully pursued among us. Dana, Draper, Ellet, Emmet, Hare, the Mitchells, Silliman, and Torrey, are well known as chemical philosophers; and Booth, Boyé, Chilton, Keating, Mather, R. Rogers, Seybert, Shepherd, and Vanuxen, as analysts; and F. Bache, Webster, Greene, Mitchell, Silliman, and Hare, as authors. In my native town of Northumberland, Pennsylvania, resided two adopted citizens, most eminent as chemists and philosophers, Priestley and Cooper. The latter, who was one of my own preceptors, was greatly distinguished as a writer, scholar, jurist, and physician, as well as a chemist. Priestley, although I do not concur in his peculiar views of theology, was certainly one of the most able and learned of ecclesiastical writers, and possessed also a mind most vigorous and original. His discoveries in pneumatic chemistry have exceeded those of any other philosopher. He discovered vital air, many new acids, chemical substances, paints, and dyes. He separated nitrous and oxygenous airs, and first exhibited acids and alkalies in a gaseous form. He ascertained that air could be purified by the process of vegetation, and that light evolved pure air from vegetables. He detected the powerful action of oxygenous air upon the blood, and first pointed out the true theory of respiration. The eudiometer, a most curious instrument for fixing the purity of air, by measuring the proportion of oxygen, was discovered by Dr. Priestley. He lived and died in my native town, universally beloved as a man, and greatly admired as a philosopher. Chemistry has actively advanced among us during the present century. Hare's compound blowpipe came from his hand so perfect, in 1802, that all succeeding attempts of Dr. Clark, of England, and of all others, in Europe and America, to improve upon it or go beyond the effects produced, have wholly failed. His mode of mixing oxygen and hydrogen gases, the instant before burning them, was at once simple, effective, and safe. The most refractory metallic and mineral substances yielded to the intense heat produced by the flame of the blowpipe. In chemical analysis, the useful labors of Keating, Vanuxen, Seybert, Booth, Clemson, Litton, and Moss, would fill many volumes. In organic chemistry, the researches of Clark, Hare, and Boyé were rewarded by the discovery of a new ether, the most explosive compound known to man. Mitchell's experiments on the penetration of membranes by gases, and the ingenious extension of them by Dr. Rogers, are worthy of all praise. The softening of indiarubber, by Dr. Mitchell, renders it a most useful article. Dyer's discovery of soda ash yielded him a competence. Our countrymen have also made most valuable improvements in refining sugar, in the manufacture of lard oil and stearin candles, and the preservation of timber by Earle's process. Sugar and molasses have been extracted in our country from the cornstalk, but with what, if any profit, as to either, is not yet determined. No part of mechanics has produced such surprising results as the steam engine, and our countrymen have been among the foremost and most distinguished in this great and progressive branch. When Rumsey, of Pennsylvania, made a steamboat, which moved against the current of the James River four miles an hour, his achievement was so much in advance of the age, as to acquire no public confidence. When John Fitch's boat stemmed the current of the Delaware, contending successfully with sail boats, it was called, in derision, the scheme boat