the tendency of the two rotations when combined with a foreign disturbance being to produce equal and opposite conical motions, the result is the same want of stability as if there was no conical motion in either direction."

Proceedings of the American Academy.-In a paper read before the American Association Meeting, at Albany, by Prof. W. B. Rogers, on the above subject, the following passage occurs:

"It always affords a sort of intellectual surprise to perceive for the first time the application of some simple and familiar mechanical principle to the grand phenomena of astronomy; to see that it is but one and the same set of laws which govern the motions of matter on earth and in the most distant regions of the heavens; to perceive a celestial phenomenon, vast in its relations both to time and space, and complex in its conditions, identified as to its mechanical cause, with the rotary movement of a little apparatus on the table before us."

A writer in the New York Tribune also discusses the principles involved in the Gyroscope as follows:

"There seems to be an inverse relation between the two motions. As the spinning motion (corresponding to the diurnal motion of a planet around its own axis) gradually dies away, the travelling motion (corresponding to the annual revolution around the sun) increases in velocity; but at first, when whirling at a very rapid rate, the mass travels around the support only about once in ten or fifteen seconds-too slowly for the 'centrifugal force' due to that (the annual) motion to be of any practical weight. The centrifugal force of the particles, due to their very rapid motion around the axis or shaft of the wheel, is of course immensely great, but as it acts equally in all directions, up, down, and laterally, it is difficult to see by what authority the weight is diminished in the least thereby; and the evident absence of any very deeplaid humbug in a toy which any one may purchase for a dollar or two and operate for himself, has seriously excited the curiosity of thousands of earnest practical men, and some space has been devoted to it in scientific and technical journals, without seeming to touch the root of the phenomenon. It has been suggested by some that the intense rapid revolution might act on the air in some way to resist the attraction of the earth; or again, that sufficient electricity might be evolved to aid in this effect; while some have such weak and confused ideas on all subjects that the simple words 'central forces' or 'planetary motion' are considered very satisfactory and full explanations.

"It will be found on trial that the experiment will succeed equally well in a vacuum, and what is still more interesting, if possible, the absolute weight of the whole is not affected in the least by the anomalous condition described. Whatever the position of the axis, as it hums gaily around with one end resting on nothing, the other extremity will be found always to pull on the string or to press on the stand with precisely the weight due to the whole. This removes all apprehensions that nature's laws are about to collapse or decay, and serves to take us a step nearer to an explanation of the secret.

"Every particle in a whirling mass tends to pull from the centre with considerable force. This, which is termed the centrifugal force of the particles, although it does not contribute one iota to diminish its weight, tends very

powerfully to prevent its change of plane. A wheel so situated does not 'lie round loose,' but every portion of its rim is very strongly pulled directly from the centre, the effect of all which is to hold it very stiffly. The 'inertia, which tends to prevent the change of position of any mass, is increased ten, or perhaps one hundred fold when the mass is set in violent rotation. The wreath in the play of 'Graces' is sent whirling in its flight, to diminish the chances of its turning itself into an edgewise position; and the obstinacy with which a mass holds its relative position when very rapidly rotated, is availed of in the rifle to keep the conical ball point foremost. The Minié ball is in form like a hay stack or sugar loaf, and the rifle is grooved so as to twist or screw the ball around at the rate of one complete revolution in every three or four feet of its path. In pitching quoits or pennies the same principle is to a certain extent availed of, to steady the projectile in its flight through the air; freely suspended within rings like a mariner's compass, a whirling mass will exhibit a strong desire to revolve constantly in the same plane, and thus to keep its axis always in the same position. If, however, such a mass be forcibly acted on by some exterior agency, and compelled to change its plane, as if, for example, one extremity of the axis be pulled to one side, the mass does not tend either to remain quietly where it is left, nor to resume its first position, but leaps suddenly towards a new position at right angles to both these. The immediate cause of this unexpected result is somewhat difficult of popular explanation, but the fact exists, and from it directly results all the peculiarities in the toy under notice.

"The effort of the unsupported end of the shaft to fall to the earth induces a horizontal twisting of the axial line, which, as one end is confined by the support, results in a revolution of the whole mass around that point. As the tendency to fall is continuous, so the tendency to revolve is of the same constant character, and this revolution, by continually changing the plane in a horizontal direction, generates another twisting effort, tending to elevate one end and depress the other. It chances that the end depressed by this force is the supported one, while the other is correspondingly lightened, and so long as the motion is sufficiently rapid to make this twisting force equal to the whole weight to be supported, it will sustain itself apparently on nothing, to the great delight of juveniles, and the profound amazement of graver thinkers. The slow travelling motion of the whole around its support is vitally essential to its operation, and it immediately displays its ordinary weight at both extremities whenever this revolution is stopped. This motion is, in short, the effect of its tendency to fall, and the cause of its tendency to climb upon its support, so that the motions and forces described mutually cause and sustain each other, and preserve of themselves their proper relations and intensities.

"The same principles are, to some degree, called into action in the spinning of a top, although the result is less remarkable and more familiar. The top gains its steady equilibrium by changing its base in diminishing circles, but is meanwhile preserved from falling immediately over by the agency first alluded to, i.e. the great force with which it resists any change of plane. If the foot of a top accidentally drops into a depression which retains it, the mass does not fall, but gyrates rapidly around at an inclination which dimi

nishes until it retains a perpendicular position. When thus constrained, the action of the top somewhat resembles the toy under notice, except in its disposition gradually to right itself, which proceeds from its being already in a position nearly perpendicular. It will be found on carefully experimenting with the new toy, that it sustains itself quietly only when started at or near a level position. If inclined upwards it acts like a top, and rights itself very gradually up until in some cases it loses its hold on the support, and falling upon the table, commences a series of circumgyrations far more vigorous than is consistent with drawing-room etiquette."

PRRMANENT REPRODUCTION OF NEWTON'S RINGS."

M. Carrère has shown to the French Academy that Newton's rings may be reproduced by letting fall on water a drop of a solution of bitumen of Judea, with benzoin and naphtha. It is a curious optical experiment, and the more so, as the film may be taken off the surface of the water on a sheet of paper, and kept, when dry, for permanent observation.

THE NAPOLEON III. SPY-GLASS.

Fig.1

The above is the name given to an ingenious contrivance by its inventor, Mr. I. Porro, a retired officer of the Piedmontese military engineers. We condense the following description from the Paris Illustration:

The improvement consists in so arranging a series of prismatic lenses that the larger portion of the spy-glass may be placed in a vertical case; as, for example, in the head of a cane. Convenience in holding, travelling, and economy of space is thus secured, while the power of the spy-glass is, in some respects, improved.

A short instrument, like that shown in fig. 2, when held in the hand, is less liable to oscillation, and enables the observer to point it correctly and steady, and to measure by means of an ocular

micrometer the distance to a given point, whenever the absolute size of the body observed is known, and vice versâ; it is also very convenient for transportation, making a pocket instrument without the usual sliding tubes, which prevent a correct centring of the lenses.

This spy-glass consists of an objective rectangular prism, fig. 1, ground in the shape of a lens on one of its catheti, and throwing back below, by reflection on its hypothenuse, the horizontal rays from the exterior body observed. These rays meet a second rectangular prism, where, by the last

reflection, they are thrown on the ocular or anterior cathetus, also shaped like a lens. The distance between the objective glass and the eye is consequently but the thickness of a prism (hardly two inches), the real length of the apparatus becomes vertical, is hidden inside the handle, which affords the observer a means to hold it in a steady position. The arrows indicate the direction in which the rays of light are reflected. The exterior shape, fig. 2, is very handy, not liable to get out of order, and the whole is quite portable, and the instrument very powerful. A is the eyeglass. B thumb screw for regulating the focus. The greatest difficulty the inventor had to contend against was to obtain perfect achromatism; in this, we are told, he has fortunately succeeded perfectly; his instruments are as free from colored spectra and aberration as the most perfect spy-glasses constructed in the ordinary manner. A small micrometer is also adapted for the purpose of computing distances.

CURIOUS USE OF THE MICROSCOPE.

Recently, on one of the Prussian railroads, a barrel which should have con tained silver coin, was found, on arrival at its destination, to have been emptied of its precious contents, and refilled with sand. On Professor Ehrenberg, of Berlin, being consulted on the subject, he sent for samples of sand from all the stations along the different lines of railway that the specie had passed, and by means of his microscope, identified the station from which the

interpolated sand must have been taken. The station once fixed upon, it was not difficult to hit upon the culprit in the small number of employées on duty there.

INVESTIGATIONS RELATIVE TO THE STEREOSCOPE.

Motion in Stereoscopic Portraits.-M. Lugeol, in making the stereoscopic portrait of one of his friends, had the idea of taking the two images or proofs one after the other, and making his friend each time look upon a different object. Thus, during the first sitting, he looked at the glass of the camera obscura, and during the second to the right at an object fixed nearly at an angle of 45 deg. These two images being placed in the stereoscope, let the observer stand opposite a window, and, without ceasing to look at the portrait, turn himself to the left or right, he will see the eyes of the portrait follow him as if they were animated. More than this has been effected by an adaptation of Sir David Brewster's natural magic toy, whose figures drawn on a circle are moved quickly round, so that three or four of them catch our eye at a particular angle, at almost the same instant, or rather at imperceptible intervals, the effect of motion is given to the limbs of the stereoscopic portrait.

On various Phenomena of Refraction through Semi-Lenses producing Anomalies in the Illusion of Stereoscopic Images.-A paper was presented at the last meeting of the British Association by Mr. Claudet, the well known photographist, having for its object to explain the cause of the illusion of curvature given to pictures representing flat surfaces, when they are examined in the refracting or semi-lenticular stereoscope. The author showed that all vertical lines seen through prisms or semi-lenses are bent, presenting their concave side to the thin edge of the prism, and as the two photographic pictures are bent in the same manner, the inevitable result of their coalescence in the stereoscope is a concave surface. The only means to avoid this defect is to employ the centre of the lenses to examine the two pictures; but as the centre does not retract laterally the two images, their coincidence cannot take place without placing the optical axis in such a position that they are nearly parallel, as if we were looking at the moon, or a very far object. This is an operation not very easy at the first attempt, but which a little practice will teach us to perform. Persons capable of using such a stereoscope will see the pictures more perfect, and all objects in their natural shape. Mr. Claudet presented to the meeting a stereoscope made on this principle, and many of the members present could see perfectly well with it. The author explained the cause of a defect which is very often noticed in examining stereoscopic pictures, viz. that the subject seems in some cases to come out of the openings of the mountings, and in some others to recede from behind-this last effect being more favorable and more artistic. Mr. Claudet recommended photographers when mounting their pictures to take care that the openings should have their correspondent vertical sides less distant than any two correspondent points of the first plane of the pictures, which could be easily done by means of a pair of compasses, measuring those respective distances. Mr. Claudet illustrated the phenomenon of vertical lines, by bent prisms forming by coalescence concave surfaces. He stated that if holding in