Notwithstanding all that has been said, and so justly said, of the notorious improvidence of the poor, it will be seen from the above hasty sketches, that they yet can and do help themselves to many things which are undeniably profitable and advantageous to them: they only want, in fact, a motive for so doing—a foregone conviction that the thing desiderated is worth having. Now, here is ground for hope—an opening, so to speak, for the point of the wedge. That the very poor may be taught to practise self-denial, in the prospect of a future benefit, these clubs have proved; and we may confess to a prejudice in their favour, not merely from what they have accomplished, but from a not unreasonable hope, that they may perchance foster a habit which will lead to far better things than even warm chimney-corners, greenwood holidays, roast geese, and plum-pudding.
In the Annuaire of the Bureau des Longitudes , recently published in Paris, appears a paper by the distinguished astronomer Arago—'On the Observations which have made known the Physical Constitution of the Sun and of different Stars; and an Inquiry into the Conjectures of the Ancient Philosophers, and of the Positive Ideas of Modern Astronomers on the Place that the Sun ought to occupy among the Prodigious Number of Stars which stud the Firmament'—in which all that appertains to the subject is so ably condensed, as to afford material for a popular summary, which we purpose to convey in the present article. The eclipse of the sun of last July, by enabling observers to repeat former observations and test their accuracy, furnished some of the results which serve to complete the paper in question, and which may be considered as settled, owing to the improvements continually taking place in the construction of instruments. Although astronomy is the exactest of sciences, its problems are not yet all fully solved; and for the determination of some of these, observers have to wait for years—in certain instances, for a century or more, until all the circumstances combine for a favourable observation. From the days of the Epicurean philosopher, who, judging from appearances, declared the sun to be no more than a foot in diameter, to those of living calculators, who give to the orb a diameter of 883,000 miles, there has been a marvellous advance. In these dimensions, we have a sphere one million four hundred thousand times larger than the earth. 'Numbers so enormous,' says M. Arago, 'not being often employed in ordinary life, and giving us no very precise idea of the magnitudes which they imply, I recall here a remark that will convey a better understanding of the immensity of the solar volume. If we imagine the centre of the sun to coincide with that of the earth, its surface would not only reach the region in which the moon revolves, but would extend nearly as far again beyond.' By the transit of Venus in 1769, it was demonstrated that the sun is 95,000,000 miles from the earth; and yet, distant as it is, its physical constitution has been determined; and the history of the successive steps by which this proof has been arrived at, forms one of the most interesting chapters in the progress of science.
It was in 1611 that Fabricius, a Dutch astronomer, first observed spots on the eastern edge of the sun, which passed slowly across the disk to the western edge, and disappeared after a certain number of days. This phenomenon having been often noted subsequently, the conclusion drawn therefrom is, that the sun is a spherical body, having a movement of rotation about its centre, of which the duration is equal to twenty-five days and a half. These dark spots, irregular and variable, but well defined on their edge, are sometimes of considerable dimensions. Some have been seen whose size was five times that of the earth. They are generally surrounded by an aureola known as the penumbra , and sensibly less luminous than the other portions of the orb. From this penumbra, first observed by Galileo, many apparently singular deductions have been made: namely, 'The sun is a dark body, surrounded at a certain distance by an atmosphere which may be compared to that of the earth, when the latter is charged with a continuous stratum of opaque and reflecting clouds. To this first atmosphere succeeds a second, luminous in itself, called the photosphere . This photosphere, more or less remote from the inner cloudy atmosphere, would determine by its outline the visible limits of the orb. According to this hypothesis, there would he spots on the sun every time that there occurred in the two concentric atmospheres such corresponding clear spaces as would allow of our seeing the dark central body uncovered.'
This hypothesis is considered by the most competent judges to render a very satisfactory account of the facts. But it has not been universally adopted. Some writers of authority have lately represented the spots as scoriæ floating on a liquid surface, and ejected from solar volcanoes, of which the burning mountains of the earth convey but a feeble idea. Hence observations become necessary as to the nature of the incandescent matter of the sun; and when we remember the immense distance of that body, such an attempt may well appear to be one of temerity.
The progress of optical science, however, has given us the means of determining this apparently insoluble question. It is well known, that physicists are enabled at present to distinguish two kinds of light—natural light and polarised light. A ray of the former exhibits the same properties on any part of its form; not so the latter. A polarised ray is said to have sides, and the different sides have different properties, as demonstrated by many interesting phenomena. Strange as it may seem, these rays thus described as having sides, could pass through the eye of a needle by hundreds of thousands without disturbing each other. Availing themselves, therefore, of the assistance of polarised light, and an instrument named the polariscope, or polarising telescope, observers obtain a double image of the sun, both alike, and both white; but on reflecting this image on water, or a glass mirror, the rays become polarised; the two images are no longer alike or white, but are intensely coloured, while their form remains unchanged. If one is red, the other is green, or yellow and violet, always producing what are called the complementary colours. With this instrument, it becomes possible to tell the difference between natural and polarised light.
Another point for consideration is, that for a long time it was supposed, that the light emanating from any incandescent body always came to the eye as natural light, if in its passage it had not been reflected or refracted. But experiment by the polariscope shewed, that the ray departing from the surface at an angle sufficiently small was polarised; while at the same time, it was demonstrated that the light emitted by any gaseous body in flame—that of street-lamps, for instance—is always in the natural state, whatever be its angle of emission. From these remarks, some idea will be formed of the process necessary to prove whether the substance which renders the sun visible is solid, liquid, or gaseous. On looking at the sun in the polariscope, the image, as before observed, is seen to be purely white—a proof that the medium through which the luminous substance is made visible to us is gaseous. If it were liquid, the light would be coloured; and as regards solidity, that is out of the question—the rapid change of spots proves that the outer envelope of the sun is not solid. On whatever day of the year we examine, the light is always white. Thus, these experiments remove the theory out of the region of simple hypothesis, and give certainty to our conclusions respecting the photosphere.
Here an example occurs of the aids and confirmations which science may derive from apparently trivial circumstances. Complaint was made at a large warehouse in Paris, that the gas-fitters had thrown the light on the goods from the narrow, and not from the broad side of the flame. Experiments were instituted, which proved that the amount of light was the same whether emitted from the broad or narrow surface. It was shewn also, that a gaseous substance in flame appears more luminous when seen obliquely than perpendicular, which explains what are known as faculæ and lucules , being those parts of the solar disk that shew themselves brighter than other portions of the surface. These are due to the presence of clouds in the solar atmosphere; the inclined portions of the clouds appearing brightest to the spectator. The notion, that there were thousands on thousands of points distinguishing themselves from the rest by a greater accumulation of luminous matter, is thus disposed of.
Читать дальше
