NEW PROOF OF THE EARTH'S ROTATION.
"The earth does move notwithstanding," whispered Galileo, leaving the dungeon of the Inquisition: by which he meant his friends to understand, that if the earth did move, the fact would remain so in spite of his punishment. But a less orthodox assembly than the conclave of Cardinals might have been staggered by the novelty of the new philosophy. According to Laplace, the apparent diurnal phenomena of the heavens would be the same either from the revolution of the sun or the earth; and more than one reason made strongly in favor of the prevalent opinion that the earth, not the sun, was stationary. First, it was most agreeable to the impression of the senses; and next, to disbelieve in the fixity of the solid globe, was not only to eject from its pride of place our little planet, but to disturb the long-cherished sentiment that we ourselves are the centre—the be-all and end-all of the universe. However, the truth will out; and this is its great distinction from error, that while every new discovery adds to its strength, falsehood is weakened and at last driven from the field.
That the earth revolves round the sun, and rotates on its polar axis, have long been the settled canons of our system. But the rotation of the earth has been rendered visible by a practical demonstration, which has drawn much attention in Paris, and is beginning to excite interest in this country. The inventor is M. Foucault; and the following description has been given of the mode of proof:
"At the centre of the dome of the Panthéon a fine wire is attached, from which a sphere of metal, four or five inches in diameter, is suspended so as to hang near the floor of the building. This apparatus is put in vibration after the manner of a pendulum. Under and concentrical with it, is placed a circular table, some twenty feet in diameter, the circumference of which is divided into degrees, minutes, &c., and the divisions numbered. Now, supposing the earth to have the diurnal motion imputed to it, and which explains the phenomena of day and night, the plane in which this pendulum vibrates will not be affected by this motion, but the table over which the pendulum is suspended will continually change its position in virtue of the diurnal motion, so as to make a complete revolution round its centre. Since, then, the table thus revolves, and the pendulum which vibrates over it does not revolve, the consequence is, that a line traced upon the table by a point projecting from the bottom of the ball will change its direction relatively to the table from minute to minute and from hour to hour, so that if such point were a pencil, and that paper were spread upon the table, the course formed by this pencil would form a system of lines radiating from the centre of the table. The practiced eye of a correct observer, especially if aided by a proper optical instrument, may actually see the motion which the table has in common with the earth under the pendulum between two successive vibrations. It is, in fact, apparent that the ball, or rather the point attached to the bottom of the ball, does not return precisely to the same point of the circumference of the table after two successive vibrations. Thus is rendered visible the motion which the table has in common with the earth."
Crowds are said to flock daily to the Panthéon to witness this interesting experiment. It has been successfully repeated at the Russell Institution, and preparations are being made in some private houses for the purpose. A lofty staircase or room twelve or fourteen feet high would suffice; but the dome of St. Paul's, or, as suggested by Mr. Sylvestre in the Times, the transept of the Crystal Palace, offers the most eligible site. The table would make its revolution at the rate of 15° per hour. Explanations, however, will be necessary from lecturers and others who give imitations of M. Foucault's ingenuity, to render it intelligible to those unacquainted with mathematics, or with the laws of gravity and spherical motion. For instance, it will not be readily understood by every one why the pendulum should vibrate in the same plane, and not partake of the earth's rotation in common with the table; but this could be shown with a bullet suspended by a silk-worm's thread. Next, the apparent horizontal revolution of the table round its centre will be incomprehensible to many, as representative of its own and the earth's motion round its axis. Perhaps Mr. Wyld's colossal globe will afford opportunities for simplifying these perplexities to the unlearned.
The pendulum is indeed an extraordinary instrument, and has been a useful handmaid to science. We are familiar with it as the time-regulator of our clocks, and the ease with which they may be made to go faster or slower by adjusting its length. But neither this nor the Panthéon elucidation constitutes its sole application. By it the latitude maybe approximately ascertained, the density of the earth's strata in different places, and its elliptical eccentricity of figure. The noble Florentine already quoted was its inventor; and it is related of Galileo, while a boy, that he was the first to observe how the height of the vaulted roof of a church might be measured by the times of the vibration of the chandeliers suspended at different altitudes. Were the earth perforated from London to our antipodes, and the air exhausted, a ball dropped through would at the centre acquire a velocity sufficient to carry it to the opposite side, whence it would again descend, and so oscillate forward and backward from one side of the globe's surface to the other in the manner of a pendulum. Very likely the Cardinals of the Vatican would deem this heresy, or "flat blasphemy."
To clearly appreciate the following popular explanation, it will be necessary for the reader to convince himself of one property of the pendulum, viz., that of constantly vibrating in the same plane. Let it be imagined that a pendulum is suspended over a common table, the parts bearing the pendulum being also attached to the table. Suppose, also, that the table can move freely on its centre like a music-stool: the pendulum being put in motion will continue to move in the same plane between the eye and any object on the walls of the room, although the table is made to revolve, and during one revolution will have radiated through the whole circumference. A few moments' reflection are only necessary to prove this.
FIGURE 1.
The above figure represents a plane or table on the top of a globe, or at the north pole of the earth. To this table are fixed two rods, from which is suspended a pendulum, moving freely in any direction. The pendulum is made to vibrate in the path a b; it will continue to vibrate in this line, and have no apparent circular or angular motion until the globe revolves, when it will appear to have vibrated through the entire circle, to an object fixed on the table and moving with it. It is scarcely necessary to say the circular motion of the pendulum is only apparent, since it is the table that revolves—the apparent motion of the pendulum in a circle being the same as the apparent motion of the land to a person on board ship, or the recession of the earth to a person in a balloon. The pendulum vibrates always in the same plane at the pole, and in planes parallel to each other at any intermediate point.