binations probably exist, which have never yet been noticed.

According to the observations of MM. Biot and GayLussac, during their aërostatic expedition, the magnetic action is not confined to the surface of the earth, but

extends into space. The moon has become highly magnetic by induction, in consequence of her proximity to the earth, and because her greatest diameter always points toward it. Her influence on terrestrial magnetism is now ascertained: the magnetism of the hemisphere that is turned toward the earth attracts the pole of our needles that is turned toward the south, and increases the magnetism of our hemisphere; and as the magnetic, like the gravitating force, extends through space, the induction of the sun, moon, and planets must occasion perpetual variations in the intensity of terrestrial magnetism, by the continual changes in their relative positions.

In the brief sketch that has been given of the five kinds of electricity, those points of resemblance have been pointed out which are characteristic of one individual power. But as many anomalies have been lately removed, and the identity of the different kinds placed beyond a doubt by Dr. Faraday, it may be satisfactory to take a summary view of the various coincidences in their modes of action on which their identity has been so ably and completely established by that great electrician.

The points of comparison are attraction and repulsion at sensible distances, discharge from points through air, the heating power, magnetic influence, chemical decomposition, action on the human frame, and lastly, the spark.

Ordinary electricity is readily discharged from points through air, but Dr. Faraday found that no sensible effect takes place from a Voltaic battery consisting of 140 double plates, either through air or in the exhausted receiver of an air-pump, the tests of the discharge being the electrometer and chemical action,-a circumstance owing to the small degree of tension, for an enormous quantity of electricity is required to make these effects sensible, and for that reason they cannot be expected from the other kinds, which are much inferior in degree.

Common electricity passes easily through rare

fied and hot air, and also through flame. Dr. Faraday effected chemical decomposition and a deflection of the galvanometer by the transmission of Voltaic electricity through heated air, and observes that these experiments are only cases of the discharge which takes place through air between the charcoal terminations of the poles of a powerful battery when they are gradually separated after contact for the air is then heated. Sir Humphry Davy mentions that, with the original Voltaic apparatus at the Royal Institution, the discharge passed through four inches of air; that, in the exhausted receiver of an air-pump, the electricity would strike through nearly half an inch of space, and the combined effects of rarefaction and heat upon the included air were such as to enable it to conduct the electricity through a space of six or seven inches. A Leyden jar may be instantaneously charged with Voltaic, and also with magneto-electricity -another proof of their tension. Such effects cannot be obtained from the other kinds, on account of their weakness only.

The heating powers of ordinary and Voltaic electricity have long been known, but the world is indebted to Dr. Faraday for the wonderful discovery of the heating power of the magnetic fluid: there is no indication of heat either from the animal or thermo electricities. All kinds of electricity have strong magnetic powers, those of the Voltaic fluid are highly exalted, and the existence of the magneto and thermo electricities was discovered by their magnetic influence alone. The needle has been deflected by all in the same manner, and magnets have been made by all according to the same laws. Ordinary electricity was long supposed incapable of deflecting the needle; M. Colladon and Dr. Faraday however have proved that, in this respect also, ordinary electricity agrees with Voltaic, but that time must be allowed for its action. It deflected the needle, whether the current was sent through rarefied air, water, or wire. Numerous chemical decompositions have been effected by ordinary and Voltaic electricity, according to the same laws and modes of arrangement. Dr. Davy decomposed water by the electricity of the torpedo,-Dr. Faraday accomplished its decomposition, and Dr. Ritchie

its composition, by means of magnetic action; and M. Botto of Turin has shown the chemical effects of the thermo-electricity in the decomposition of water, and some other substances. The electric and galvanic shock, the flash in the eyes, and the sensation on the tongue, are well known. All these effects are produced by magneto-electricity, even to a painful degree. The torpedo and gymnotus electricus give severe shocks, and the limbs of a frog have been convulsed by thermo-electricity. The last point of comparison is the spark, which is common to the ordinary Voltaic and magnetic fluids; and Professor Linari, of Siena, has very lately obtained both the direct and induced sparks from the torpedo, proving that in this respect animal electricity does not differ from the others. Indeed, the conclusion drawn by Dr. Faraday is that the five kinds of electricity are identical, and that the differences of intensity and quantity are quite sufficient to account for what were supposed to be their distinctive qualities. He has given still greater assurance of their identity by showing that the magnetic force and the chemical action of electricity are in direct proportion to the absolute quantity of the fluid which passes through the galvanometer, whatever its intensity may be.

In light, heat, and electricity, or magnetism, nature has exhibited principles which do not occasion any appreciable change in the weight of bodies, although their presence is manifested by the most remarkable mechanical and chemical action. These agencies are so connected, that there is reason to believe they will ultimately be referred to some one power of a higher order, in conformity with the general economy of the system of the world, where the most varied and complicated effects are produced by a small number of universal laws. These principles penetrate matter in all directions; their velocity is prodigious, and their intensity varies inversely as the squares of the distances. The development of electric currents, as well by magnetic as electric induction, the similarity in their mode of action in a great variety of circumstances, but above all, the production of the spark from a magnet, the ignition of metallic wires, and chemical decomposition, show that

magnetism can no longer be regarded as a separate independent principle. Although the evolution of light and heat during the passage of the electric fluid may be from the compression of the air, yet the development of electricity by heat, the influence of heat on magnetic bodies, and that of light on the vibration of the compass, show an occult connection between all these agents, which probably will one day be revealed. In the mean time it opens a noble field of experimental research to philosophers of the present, perhaps of future ages.

SECTION XXXVI.

Ethereal Medium-Comets-Do not disturb the Solar System-Their Orbits and Disturbances-M. Faye's Comet, probably the same with Lexel's-Periods of other three known-Halley's-Acceleration in the Mean Motions of Encke's and Biela's Comets-The Shock of a CometDisturbing Action of the Earth and Planets on Encke's and Biela's Comets-Velocity of Comets-The Great Comet of 1843-Physical Constitution-Shine by borrowed Light-Estimation of their Number.

IN considering the constitution of the earth and the fluids which surround it, various subjects have presented themselves to our notice, of which some, for aught we know, are confined to the planet we inhabit; some are common to it and to the other bodies of our system. But an all-pervading ether probably fills the whole visible creation, and conveys, in the form of light, tremors which may have been excited in the deepest recesses of the universe thousands of years before we were called into being. The existence of such a medium, though at first hypothetical, is nearly proved by the undulatory theory of light, and rendered all but certain within a few years by the motion of comets, and by its action upon the vapors of which they are chiefly composed. It has often been imagined, that, in addition to the effects of heat and electricity, the tails of comets have infused new substances into our atmosphere. Possibly the earth may attract some of that nebulous matter, since the vapors raised by the sun's heat, when the comets are in perihelio, and which form their tails, are scattered through space in their passage to their aphelion; but it has hitherto produced no effect, nor have

the seasons ever been influenced by these bodies. The light of the comet of the year 1811, which was so brilliant, did not impart any heat even when condensed on the bulb of a thermometer, of a structure so delicate that it would have made the hundredth part of a degree evident. In all probability, the tails of comets may have passed over the earth without its inhabitants being conscious of their presence; and there is reason to believe that the tail of the great comet of 1843 did so.

The passage of comets has never sensibly disturbed the stability of the solar system; their nucleus, being in general only a mass of vapor, is so rare, and their transit so rapid, that the time has not been long enough to admit of a sufficient accumulation of impetus to produce a perceptible action. Indeed M. Dusejour has proved, that under the most favorable circumstances, a comet cannot remain longer than two hours and a half at a less distance from the earth than 10,500 leagues. The comet of 1770 passed within about six times the distance of the moon from the earth, without even affecting our tides. According to La Place, the action of the earth on the comèt of 1770 augmented the period of its revolution by more than two days; and if comets had any perceptible disturbing energy, the reaction of the comet ought to have increased the length of our year. Had the mass of that comet been equal to the mass of the earth, its disturbing action would have increased the length of the sidereal year by 21 53m; but as Delambre's computations from the Greenwich observations of the sun show that the length of the year has not been increased by the fraction of a second, its mass could not have been equal to the 5th part of that of the earth. This accounts for the same comet having twice swept through the system of Jupiter's satellites without deranging the motion of these moons. M. Dusejour has computed that a comet, equal in mass to the earth, passing at the distance of 12,150 leagues from our planet, would increase the length of the year to 367d 16h 5m, and the obliquity of the ecliptic as much as 2°. So the principal action of comets would be to alter the calendar, even if they were dense enough to affect the earth.

Comets traverse all parts of the heavens; their paths