given by Foucault in 1850. There was one important point on which these rival theories led to directly opposite conclusions, and in 1839, Arago proposed this point as the basis of an experimentum crucis. According to the emission theory, light would traverse dense media more rapidly than it would air or vacuum. The wave theory gives as a result, that light, in traversing dense masses, must be retarded. This antagonism was positive and one theory must be erroneous. But which one? To answer this query, Foucault devised an ingenious apparatus, based on Wheatstone's revolving mirror. Two similar beams of light, in the same vertical plane, were made to pass, one through a long column of water, and one though the air, so as both to fall on the same vertical line of a plane mirror, so attached to a vertical axis as by its means to be made to revolve in horizontal circles with extreme rapidity. Now, the light of both beams starting simultaneously would reach the mirror at successive instants, if one beam were accelerated or retarded, and thence the two reflected flashes would be seen, not vertically above each other, but one would seem in advance. If the water-beam be accelerated, it will be first reflected, and if it be retarded, it will be last reflected, and so would seem to be carried around with the revolving mirror. Foucault tried the experiment in various forms, and the result always indicated an unmistakable retardation of the beam in the water. Hence, the emission theory mathematically leads to a false result, and must itself be erroneous. That the wave theory is a true generalization is now conceded by all competent authorities. It rests on proof of the same nature as does the Newtonian law of gravitation, and this proof has now accumulated to such an extent as to make the demonstration irresistible on all the main features. The unequal refrangibility of the different colored rays alone resists, and, despite the profound researches of Cauchy on this point, we think the truth is not yet all told. Henceforth, however, no man has better reason for advocating the emission theory than for arguing that one plus one make eleven.

We now come to Foucault's famous pendulum experiment, by which he set all the world to bobbing. The elephants and tortoises had long been re

tired from active duty as earth support. ers, and the sky had been lifted from the shoulders of Atlas. That the earth revolved about the sun and its own axis was no longer a heresy punishable by roasting alive. Among those professing science, no danger to mill-ponds was apprehended from their being daily turned bottom side up. The demonstration of the earth's rotation lacked nothing of completeness to such as could follow the proof. Yet, that the earth revolved had not been experimentally made a separately visible thing, and, perhaps, there were some doubting Thomases, who must needs see before believing. Foucault's pendulum experiment must convince even them; for it makes the steady rolling of the earth around its axis, a visible phenomenon, minute by minute, even as the clockhands indicate the flight of time. First privately, and then at the Paris observatory, Foucault tried his experiment. Finally, the 220 feet pendulum at the Pantheon, with its majestically slow sweeps, exhibited to enthusiastic Paris that the earth turned under the fixed arc of vibration. Thence, throughout the world, a pendulum mania extended, until a monster pendulum threatened to become essential to every respectable household.

We may, perhaps, aid in giving a clear idea of the rationale of this experiment by a simple exposition. Suppose a circular railroad of a hundred feet radius, around which a car is uniformly traveling, on which car, a heavy beam, supported, at its centre of gravity, like a compass needle, is left otherwise wholly free. This beam (if friction and air resistance be nullified) will, instead of turning with the car, remain all the while parallel to itself, simply because no force of rotation would thus be applied to it. To an observer in the car, it would seem to revolve once for each circuit of the car. If this beam be replaced by a pendulum, suspended to the top of the car and vibrating in a given plane, then all the succeeding planes of vibration will be parallel to this plane, because no force would be applied tending to whirl these planes of vibration with the car.

Had we an indefinite

plane surface, towards which gravity acted perpendicularly, and a circular railroad of any radius thereon, all the above appearances would present themselves unchanged. Suppose this ra

dius equal to the horizontal tangent line, passing due north from our position on the earth to the point where this line cuts the earth's axis-i. e., equal to the cotangent of our latitude. Suppose, now, this circle cut out of the plane, and split along one of its radius lines to the centre. Proceeding as if we were about making a huge cone for bon-bons, suppose the cut radius edges overlapped and closed in till the radius of the base of the cone thus formed bears the same proportion to the railroad radius that the sine of our latitude bears to the earth's radius. This cone will be precisely like the tangent cone to the earth along our parallel of latitude. Now, in this rolling up of our railroad into a circle on a cone, we suppose the action of gravity always perpendicular to the surface at each point of its area; hence no change is effected in the mechanical conditions of the pendulum, but it will still operate the same, relative to a fixed line in the car, or its plane of vibration will still seem to a passenger to be whirling. But in going once around the cone, it will only go partly around the original railway circle-the proportion being as the sine of the latitude to the earth's radius, which is also the portion of a complete revolution that the plane of vibration would seem, to a passenger, to make during one circuit of the parallel. This reproduces the essential mechanical conditions of our daily revoIution around the earth's axis; for, obviously, the forces acting on the pendulum have nothing to do with the earth's rotation, except as that transports the pendulum. Hence, the plane of vibration of a freely suspended pendulum must appear to revolve around the vertical through its point of suspension, at a rate equal, daily, to one revolution multiplied by the sine of the latitude. At the pole, where this sine becomes equal to the radius, a complete daily revolution would take place; but at the equator our circular railroad would have an infinite radius and no rotation of the vibrating plane would appear. Such is the simple rationale of the famous pendulum experiment.

Innumerable trials have vindicated this theory and have verified the law of variation with the sine of the latitude. Many modifications of this pendulum device have been proposed, and one of much beauty has been successfully tried. It occurred to Bravais that the

gyrations of a conical pendulum, or one whirling like the arms of a steam-engine governor so as to describe a cone, would be affected by the earth's rotation, and on trial he found his theory correct. The second pendulum at Paris, revolving conically, would lose three seconds a day, when whirling from right to left, and would gain the same when whirling from left to right. No result of the pendulum crusade possessed more elements of interest than the discovery made by the excellent and ingenious Prof. Horsford of Cambridge, relative to the diurnal movements of Bunker Hill monument, due to the successive heating actions of the sun's rays on the different sides. Daily, the summit of that proud pile was found to trace an approximate elliptic orbit of about half an inch major axis, offering thus an homage to the great luminary, far more grand than the moaning wail of the old Memnonian statue. It is as if Webster's eloquent apostrophe had entered the very stones.

We have next to mention Foucault's device called the gyroscope; another distinct and admirable method of making the earth's rotation visible. Nearly all are familiar with the gimbal mounting of ship's compasses, to keep them level and steady during the rolling of the vessel. Foucault arranged a heavy torus disc or wheel with an axis of revolution through its centre of gravity, as illustrated in an ordinary grindstone. Having, by a separate arrangement, given a very rapid rotation to this disc, it is transferred, without checking its whirling, to a delicate gimbal mounting, where the rotation continues a long time. This disc is free from the usual constraining forces of common bodies, and there is nothing to prevent its plane of rotation from maintaining the same species of parallelism as the pendulum planes of vibration do. Hence, to an observer, the disc seems to turn on its mountings at a rate varying with the sine of the latitude; and this apparent motion indicates the real rotation of the earth on its axis. An understanding of the mechanical theory of rotations must precede a full comprehension of gyroscope actions; hence this method of exhibiting the earth's rotation, though more subtle and perfect than that by the pendulum, has achieved less notoriety.

Foucault's last announcement in September, 1855, makes known a truly

elegant and significant discovery. It trenches on the most recondite fields of investigation, and gives unlimited promise of future results. The facts are exceedingly simple, but the deductions reach beyond our present imaginings.

On placing the disc of the gyroscope between, and in close proximity with, the poles of an electro-magnet, and giving to this disc a rotation of from 150 to 200 turns per second, it is found that immediately after closing the electric circuit and bringing the electro-magnet into action, the rotation becomes restrained by a constant resistance, as if an invisible friction-brake were applied, until the rotation is entirely checked. So far the discoveries of Arago and Faraday had virtually extended. But now Foucault has found that if force be applied to restore the motion, there is a steady development of heat in the disc, dependent on the action of the electromagnet. This elevation of temperature was so marked that, the hand readily perceived it, and, indeed, it ranged from 16° centigrade as high as 40°. The current used was only that from six of Bunsen's couples. By enlarging the battery, and increasing the rapidity of rotation, the results can, doubtless, be intensified quite considerably. In this experiment, we have but to break the circuit and the heating effects disappear.

The significance of this discovery lies in its indication of close relations be tween the electric current, heat, and mechanical force or labor. They seem to be absolutely convertible into each other. Faraday has succeeded in changing the plane of polarization of light, by subjecting a polarized beam to intense magnetic action. Each year is bringing to light new illustrations of the correlation of all physical forces or agencies, none of which, however, is so remarkable as this last development by Foucault. It possesses special import

ance, at this time, in its bearings on the doctrine of mechanical equivalents of heat which has recently engaged great attention. In its broadest statement, this doctrine declares all natural forces or agencies to be a true unity, and teaches that, when heat, electro-magnetism, gravity, elasticity, friction, muscular action, or, indeed any type of active or passive force is transformed into any other type, it is changed into a certain equivalent amount of the new force, which amount is definite and constant, and that all these force equivalents are in mutual correspondence. The analogy of chemical equivalents may serve to illustrate this formula. Joule has developed the whole doctrine experimentally, and Prof. Tomson has given its analytical exposition. Prof. Barnard of Alabama and Maj. Barnard of the U. S. Engineers, have with signal success applied this theory to the discussion of the caloric or air-engine, especially as combined by Ericsson. A clear apprehension of this fundamental view, might have saved all concerned their forlorn descent from the towering pinnacle of promise to the humble vale of achievement. The production and application of mechanical power are governed by laws too rigorous to permit of legerdemain. Whatever will improve the economy of generating, transferring, or applying motive power may achieve success; but rash ventures in schemes based on the "higher law," are little to be commended. Foucault has contributed much to the proof that all force has a unity of nature, by this simple experimental conversion of electrodynamic action into heat, itself result ing from mechanical or muscular force. We hail in it the twilight of a better philosophy concerning those subtle agencies which underlie all the workings of nature, and which, by eluding our senses, appeal to the nobler mind within us for their better interpretation.

AT the same house where I lived at

Florence, on the same floor, in a room further down the passage, lodged a girl of eighteen or nineteen, slender, generally pale, but with flashing black eyes, and features that were, on the whole, rather pretty. She called herself Maria, was a native of Sienna, and followed the trade of seamstress. As I saw her nearly every day for over three months, we had plenty of opportunities for the exchange of ideas, and I found her an invaluable mine of Italian idioms and Italian credences. For, be it known that Sienna is the place, of all Italy, where the language is the purest; and not only this, but it is a retired little city, somewhat away from the great lines of travel and thought; so that old opinions and superstitions still possess there a perceptible degree of vitality. I collected a small museum of ghostfrights and witch-adventures from Maria's conversation; although such was her timidity and fear of ridicule, that she would only relate these wonders by dint of being delicately coaxed and managed.

She was the most bashful girl that I ever saw-bashful with a kind of nervousness, bashful even to disease-and to the last, she never looked me full in the eyes for more than a lightning-like glance. Her face bent down so as to be half hidden by its own shadow, or by one hand lifted partially over it, she used to prattle Florentine gossip, or relate her astonishing histories. If I looked incredulous, above all, if I laughed, she would stop and declare, with a comical pettishness, that she never would tell me another word.

"Witches? O yes! to be sure there were witches. Why, her mother had told her how-" and here catching a smile upon my face, she came to an indignant stop. "There; now you are laughing at me. I knew you would laugh; I knew you would not believe it. I will not tell you anything more. You shall not laugh at me.'

"No, no, Maria. I am not laughing now. Tell me all about it; perhaps I shall believe it. Just tell me the story, and let me see what I think of it."

Then came a most ridiculous narration, how her mother, when a girl, was

very handsome, and thus attracted the evil eye of an old woman of Sienna, who had the name of being a witch. This old woman offered her mother an apple, and pressed it upon her so urgently, that, against her better judgment, she accepted and ate it. Consequently, her mother fell sick, and pined away in such a manner as very much astonished her relatives, until they learned the adventure of the apple, when they immediately understood the cause of her illness. Then her father and brothers went to the old woman's house, and, surrounding her with their knives drawn, said: "Thou hast bewitched our daughter and sister, and deservest to die; but cure her and thou shalt live, and we will promise secrecy concerning thy crime."

So the old woman, in a great fright, went to the chamber of the invalid, and anointed her with some species of ointment which she caused to be prepared for the occasion; after which she kneaded her from head to foot, as you would knead bread, and so brought her out to the family, as smooth, and sound, and handsome as ever. The father and brothers kept their agreement of silence until the witch died, when they felt at liberty to repeat the tale, which had ever since been a current thing at Si

enna.

There was a better story of a poor woman who fell partially into the power of Satan through an evil wish. The night following this crime of thought, she was awaked by a tap on the window; and, looking through the glass, she saw a goat which motioned her with one of its fore-hoofs to come out. She was under the influence of some terrible charm; for she neither dared wake her husband, nor keep her place; and so, rising, she slipped noiselessly through the door, and stood before the strangely potent animal. "Wilt thou harm Christ's earth, or his followers ?" said the goat.

"I will harm the earth," said the woman, who already repented of her sin, and had no desire to injure her fellowcreatures.

Then mount on my back," replied the goat. The woman was so constrained by some mysterious power to obey,

that she instantly bestrode the animal, unable to take any other precaution than to cling to its long hair. Immediately the goat went off with the swiftness of wind; springing along the bending surface of the cornfields; leaping from festoon to festoon in the vineyards, and galloping madly over the top of the trees. Wherever his feet struck, they ruined everything; crushing the grain to earth; tearing the vines in pieces; splintering to the roots the strongest olives and mulberries. The miserable rider was bruised and wounded by the crashing branches; her thin robe torn from her in shreds, and her strength exhausted by fatigue and terror; until, after an hour of this fearful aerial gallop, she was brought back to her own door and flung violently from the infernal animal's back. There she lay breathless, unable to move, and with a fearful enchantment upon her of which even she

was unaware.

Morning came, and the husband, not beholding his spouse, first called her, then hunted the house over, and finally sought her out of doors. He saw a huge unsightly toad on the threshold, and indignantly kicked it into the bushes. No wife being anywhere discoverable, he hurried to the neighbors and told them of this incomprehensible disappearance of his rib. Of course the poor man's hearth was soon inundated by an assembly of curious gossips, among whom was the pious old priest of the village. As the holy father trotted about the house, peeping into the most improbable localities for finding a woman, he happened to spy, nestled among the bed-clothes, a toad of extraordinary magnitude; so prodigious, in fact, and so abominably ugly, that, in his amazement at the sight of it, the good man incontinently said a benedicite. The moment the sacred words were pronounced, the toad changed shape and became the mistress of the house, who immediately proceeded, with many tears and faintings, to tell her lamentable story. She was still dreadfully scratched and pounded from her midnight ride; and had a large bruise on her cheek, caused by the heavy toe of her husband's shoe ; so that they were constrained to leave her. The wise father immediately took all the necessary precautions against a second visit from the devil; blessing the house, sprinkling holy water copiously about the grounds, and holding espe

cial service in the parish church that afternoon. These vigorous measures were, by the favor of the Madonna, perfectly successful; and the fiendish goat never troubled the family thereafter with his nocturnal visits.

Another of Maria's stories struck me as really pleasing, and as affording a subject for a pretty night-picture. She said that a pious, poor man of Sienna went into the church of San Francisco to say his evening prayers; and, being very tired, sat down on a bench against the wall, where he presently fell asleep. The vespers ended; the worshipers passed out; the sexton closed the dim church; yet the sleeper remained at his post. He was awakened at midnight by a glare of light falling across his eyelids. Greatly astonished to find himself napping in so holy a place at such an hour, he was still more amazed at seeing the altar-candles alight with a halo like that around the head of Christ in pictures, while before them a priest, in white robes, was in the act of commencing a mass. But, being a man of pious disposition, and, also, not a little awed by the circumstances in which he found himself, he very reverently joined in the service, making the usual responses, and bowing his knees at the proper time. The priest recited with extraordinary fervor; and our Siennese felt unusually edified and uplifted by the holy words; more so

than had ever been the case with him on any previous religious occasion. The mass being ended, the priest noise lessly glided to the sacristy, and entered it, without drawing the curtain, or even shaking it by his passage. The spectator hesitated some time between respect and curiosity, but finally stole to the doorway, and, cautiously pushing aside the drooping linen, peeped into the sacerdotal precinct.

At that moment the priest rose from his knees, with a countenance full of unearthly joy, and turned towards him. The Siennese would have drawn back; but when the other in a low, sweet tone bade him enter, he obeyed, and stood trembling by the door.

"My son," said the priest, "thou art anxious to know why I celebrate this service alone and at this unusual hour. Know, then, that I am a spirit just liberated from purgatory, and by thy means. When I died I had one grievous sin on my soul; and that was