« PreviousContinue »
produce phosphorescent light, which is extinguished by the least refrangible luminous rays. According to Mr. Hunt's experiment, the hot rays condense mercurial vapour on a polished metallic plate, while the luminous rays prevent its formation. Electricity is excited by the chemical rays, while the parathermic are found in the less refrangible rays alone. Each of the spectra is crossed by coloured and rayless lines peculiar to itself, and these are traversed at right angles by innumerable dark lines of various breadths, the whole forming an inexpressibly wonderful and glorious creation.
The arrangement varies a little according to the material of the prism and the manner of producing the spectrum, as in that obtained by Professor Draper from diffracted light. It was formed by a beam diffracted by passing through a netting of fine wire, or by reflection from a polished surface of steel, having fine parallel lines drawn on it. This diffracted spectrum is divided into two equal parts in the centre of the yellow; and as in the prismatic spectrum, one half is antagonist to the other half, the red or negative end undoing what the positive or violet end has done. The centre of the yellow is the hottest part, and the heat decreases to both extremities. A line of cold is supposed to exist on this spectrum answering to Fraunhofer's dark line H.
The undulations of the ethereal medium which constitute a sunbeam must be infinitely varied, each influence having a vibration peculiar to itself. Those of light are certainly transverse to the direction of the ray; while Professor Draper believes that those of heat are normal, that is, in the direction of the ray, like those of sound. A doubt exists whether the vibrations of polarised light are perpendicular to the plane of polarisation or in that plane. Professor Stokes of Cambridge has come to the conclusion, both from the diffracted spectrum and theory, that they are perpendicular to the plane of polarisation, but M. Holtzmann is of opinion that they are in that plane, so the subject is still open to discussion.
Size and Constitution of the Sun-The Solar Spots - Intensity of the Sun's Light and Heat - The Sun's Atmosphere -- His influence on the Planets Atmospheres of the Planets - The Moon has none- Lunar heat-The Differential Telescope - Temperature of Space - Internal Heat of the Earth Zone of constant Temperature- Increase of Heat with the Depth - Central Heat - Volcanic Action - Quantity of Heat received from the Sun - Isogeothermal Lines Line of perpetual Congelation Climate Isothermal Lines - Same quantity of Heat annually received and radiated by the Earth.
THE sun is a globe 880,000 miles in diameter : what his body may be it is impassible to conjecture, but it seems to be a dark mass surrounded by an extensive atmosphere at a certain height in which there is a stratum of luminous clouds which constitutes the photosphere of the sun. Above it rises the true solar atmosphere, visible as an aureola or corona during annular and total eclipses, and probably the cause of the peculiar phenomena in the photographic image of the sun already mentioned. Through occasional openings in the photosphere or mottled ocean of flame, the dark nucleus appears like black spots, often of enormous size. These spots are almost always comprised within a zone of the sun's surface, whose breadth measured on a solar meridian does not extend beyond 30° on each side of his equator, though they have been seen at a distance of 3910. The dark central part of the spots is surrounded by a succession of obscure cloudy envelopes increasing in brightness up to a penumbra, sometimes there are three or more shades, but it requires a good telescope to distinguish the intermediate ones. The spots gradually increase in size and number from year to year to a maximum, and then as gradually decrease to a minimum, accomplishing regular vicissitudes in periods of about eleven years, and are singularly connected with the cycles of terrestrial magnetism. From their extensive and rapid changes, there is every reason to believe that the exterior and incandescent part of the sun is gaseous.
Doubts have arisen as to the uniformity of the quantity of
heat emitted by the sun. Sir William Herschel was the first to suspect that it was affected by the quantity and magnitude of the spots on his surface; Professor Secchi has observed that the spots are less hot than the luminous part; and now Professor Wolf has perceived that the amount of heat emitted by the sun varies periodically with the spots every 11.11 years, or nearly nine times in a century, beginning at the commencement of the present one. He has discovered a sub-period in that of the spots, which no doubt has an effect on the quantity of solar heat. So the unaccountable vicissitudes in the temperature of different years may ultimately be found to depend upon the constitution of the sun himself.
The intensity of the sun's light diminishes from the centre to the circumference of the solar disc. His direct light has been estimated to be equal to that of 5563 wax candles of moderate size placed at the distance of one foot from an object; that of the moon is probably only equal to the light of one candle at the distance of 12 feet: consequently the light of the sun is more than three hundred thousand times greater than that of the moon. According to Professor Secchi's experiments at Rome, the heat of the solar image is almost twice as great at the centre as at the edge. The maximum heat, however, is not in the centre, but in the solar equator, and the spots are less hot than the rest of the surface.
The oceans of light and heat probably arising from electric or chemical processes of immense energy that continually take place at the sun's surface (N. 217) are transmitted in undulations by the ethereal medium in all directions; but notwithstanding the sun's magnitude and the inconceivable intensity of light and heat that must exist at his surface, as the intensity of both diminishes as the square of the distance increases, his kindly influence can hardly be felt at the boundaries of our system. In Uranus the sun must be seen like a small brilliant star not above the hundred and fiftieth part as bright as he appears to us, but that is 2000 times brighter than our moon, so that he is really a sun to Uranus, and may impart some degree of warmth. But if we consider that water would not remain fluid in any part of Mars, even at his equator, and that, in the temperate zones of the same planet, even alcohol and quicksilver would freeze, we may form some idea of the cold that must reign in Uranus and Neptune.
The climate of Venus more nearly resembles that of the earth, though, excepting at her poles, much too hot for animal and vegetable life such as they exist here, for she receives seven times as much light and heat as the earth does; but in Mercury the mean heat from the intensity of the sun's rays must be above that of boiling quicksilver, and water would boil even at his poles. Thus the planets, though kindred with the earth in motion and form, are, according to our experience, totally unfit for the habitation of such a being as man, unless indeed their temperature should be modified by circumstances of which we are not aware, and which may increase or diminish the sensible heat so as to render them habitable. In our utter ignorance it may be observed, that the earth, if visible at all from Neptune, can only be a minute telescopic object; that from the nearest fixed star the sun must dwindle to a mere point of light; that the whole solar system would there be hid by a spider's thread; and that the starry firmament itself is only the first series of starry systems, the numbers of which are bounded alone by the imperfection of our space-penetrating instruments. In this overwhelming majesty of creation, it seems rash to affirm that the earth alone is inhabited by intelligent beings, and thus to limit the Omnipotent, who has made nothing in vain.
Several of the planets have extensive and dense atmospheres: according to Schroëter the atmosphere of Ceres is more than 668 miles high, and that of Pallas has an elevation of 465 miles, but not a trace of an atmosphere can be perceived round Vesta. The attraction of the earth has probably deprived the moon of hers, for the refractive power of the air at the surface of the earth is at least a thousand times as great as at the surface of the moon: the lunar atmosphere must therefore be of a greater degree of rarity than can be produced by our best air-pumps. This is confirmed by Arago's observations during a solar eclipse, when no trace of a lunar atmosphere could be seen. Since then, however, some indications of air have been perceived in the lunar valleys. In taking photographic images of the moon and Jupiter at Rome, Professor Secchi found that the light of the full moon is to that of the quarter moon as 3 to 1. Jupiter gives a photographic image as bright and vigorous as the brightest part of the moon; but although the light of Jupiter is less than that of the moon, he is nearly five times farther from the sun; and as light
diminishes as the square of the distance increases, the light of Jupiter is proportionally greater than that of the moon, consequently Jupiter's atmosphere reflects more light than the dark volcanic soil of the moon; thus Professor Secchi observes photography may in time reveal the quality of the materials of which the celestial bodies are formed.
The effect of the earth's atmosphere on lunar heat is remarkable. Professor Forbes proved that the direct light of the full moon is incapable of raising a thermometer the one three thousandth part of a Centigrade degree, at least in England; but at an elevation of 8870 feet on the Peak of Teneriffe, Mr. Piazzi Smyth found a very sensible heat from the moon, although she was then 19° south of the equator; so it is no doubt absorbed by our atmosphere at lower levels.
Some exceedingly interesting experiments might be made by means of a telescope having a prism attached to its objective extremity, and furnished with a micrometer, because by it the difference of the illumination of objects might be determined with extreme accuracy-as for example, the comparative intensity between the bright and dark parts of the moon, the comparative intensity of the solar light reflected by the moon, and the lumière cendré, or the light of the earth reflected on the moon, whence a comparison might be made between the light of the sun and that of the earth. Hence also it might be known whether the terrestrial hemispheres successively visible from the moon are more or less luminous, according as they contain more land or water, and at the same time it might be possible to appreciate the more or less cloudy or clear state of our atmosphere, so that in time we might ultimately find in the lumière cendré of the moon data upon the mean diaphaneity of different terrestrial hemispheres which are of different temperatures.
It is found by experience that heat is developed in opaque and translucent substances by their absorption of solar light, but that the sun's rays do not sensibly alter the temperature of perfectly transparent bodies through which they pass. As the temperature of the pellucid planetary space can be but little affected by the passage of the sun's light and heat, neither can it be sensibly raised by the heat now radiated from the earth.
Doubtless the radiation of all the bodies in the universe maintains the ethereal medium at a higher temperature than it would