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
This address was made and published several months before any electric telegraph line was in operation, and is believed to be the first prediction of the success of this principle, as Continental or Oceanic.
