ALTHOUGH the subject of lunar eclipses has already been discussed, it is again briefly noticed because it forms one of the category of supposed evidences of the earth's rotundity. Those who hold that the earth is a globe will often affirm, with marked enthusiasm, that in an eclipse of the moon there is "proof positive" of rotundity. "Is not the shadow of the earth, on the moon, always round?" "Could anything but a globe cast a shadow which at all times, and in all positions, is visibly circular?" "Would
not a plane sometimes cast a shadow edgewise, which, on the moon, would appear as a bar or straight line across it?" Notwithstanding the plausibility of these questions, the essential requirements of an argument are wanting. That the eclipsor of the moon is a shadow at all is assumption--no proof whatever is offered. That the moon receives her light from the sun, and that therefore her surface is darkened by the earth intercepting the sun's light, is not proved. It is not proved that the earth moves in an orbit round the sun, and therefore, by being in different positions, conjunction of sun, earth, and moon, 'Day some-times occur. The contrary has been clearly proved--that the moon is not eclipsed by a shadow; that she is self-luminous, and not merely a reflector of solar light, and therefore could not possibly be obscured or eclipsed by a shadow from any object whatever; and that the earth is devoid of motion, either on axes or in an orbit through space. Hence to call that an argument for the earth's rotundity, where every necessary proposition is only assumed, and in relation to which direct and practical evidence to the contrary is abundant, is to stultify the judgment and every other reasoning faculty.
Thus we have seen that in every instance where the attempt is made to prove the rotundity of the earth, the premises do not warrant the conclusion, which is premature--drawn before the whole subject is fairly stated and examined, and when other and visible causes are amply sufficient to explain the phenomena, for explanation of which the theory of rotundity was originally framed.
The same charge may be made against the few instances
which have been adduced as proofs of the earth's motion. To explain day and night, the earth was assumed to revolve once in twenty-four hours. The only direct proofs offered are the peculiarities attending the oscillations of a long pendulum, and the tendency of railway carriages to be thrown off the rails when running on lines in a due northerly or southerly direction. In the early part of the year 1851 the scientific journals, and nearly all the news-papers published in Great Britain, and on the continents of Europe and America, were occupied in recording and discussing certain experiments with the pendulum, first made by M. Foucault, of Paris; and the public were startled by the announcement that the results furnished a practical proof of the earth's rotation. The subject was referred to in the Literary Gazette in the following words:--
"Everybody knows what is meant by a pendulum in its simplest form, a weight hanging by a thread to a fixed point. Such was the pendulum experimented upon long ago by Galileo, who discovered the well-known law of isochronous vibrations, applicable to the same. The subject has since received a thorough examination, as well theoretical as practical, from mathematicians and mechanicians; and yet, strange to say, the most remarkable feature of the phenomenon has remained unobserved and wholly unsuspected until within the last few weeks, when a young and promising French physicist, M. Foucault, who was induced, by certain reflections, to repeat Galileo's experiments in the cellar of his mother's house at Paris, succeeded in establishing the existence of a fact connected with it, which gives an immediate and visible demonstration of the earth's rotation.
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[paragraph continues] Suppose the pendulum already described to be set moving in a vertical plane from north to south; the plane in which it vibrates, to ordinary observation, would appear to be stationary. M. Foucault, however, has succeeded in showing that this is not the case, but that the plane is itself slowly moving round the fixed point as a centre, in a direction contrary to the earth's rotation, i.e., with the apparent heavens, from east to west. His experiments have since been repeated in the hall of the observatory, under the superintendence of M. Arago, and fully confirmed. If a pointer be attached to the weight of a pendulum suspended by a long and fine wire, capable of turning round in all directions, and nearly in contact with the floor of a room, the line which this pointer appears to trace on the ground, and which may easily be followed by a chalk mark, will be found to be slowly, but visibly, and constantly moving round, like the hand of a watch dial. . . . The subject has created a great sensation in the mathematical and physical circles of Paris.
"It is proposed to obtain permission from the Government to carry on further observations by means of a pendulum suspended from the dome of the Pantheon, length of suspension being a desideratum, in order to make the result visible on a larger scale, and secure greater constancy and duration in the experiments."
Subsequently experiments were made at the Pantheon, and repeated in almost every part of the civilised world, but with results so variable, and in many instances the very contrary to the anticipations suggested by theory, that many of the same Newtonian school of philosophy differed with each other, remained dissatisfied, and raised very serious objections both to the value of the experiments
themselves, and to the supposed proof which they furnished of the earth's rotation. One writer in the Times newspaper of the period, who signs himself "B. A. C.," says:--
"I have read the accounts of the Parisian experiment, as they have appeared in many of our papers, and must confess that I still remain unconvinced of the reality of the phenomenon."