« السابقةمتابعة »
sharply defined, so that a micrometer may almost unerringly determine the angle it subtends; on the contrary, á cometary disc, from the nebulous matter through which it is seen, is generally rough and indistinct. Radiation, also, will cause the nucleus of a comet to appear larger than it really is, which is evident from the fact, that stars seen through the nebulous matter appear larger, though not brighter than when escaped from the vapoury medium. The comet of Halley was calculated to be about the magnitude of the moon, that of 1680 was supposed to be ten times as large as the moon; those comets which present merely a mass of nebulosity without a nucleus, vary in their sizes from that of 1804, which was 5000 miles in diameter, to that of the Encke comet, which, on its last return in 1828, was found to be 76000 miles in diameter ; but it must be remembered, that these last mentioned contain very little matter in proportion to their magnitudes, being probably of not much greater density than the tails of some comets : this may be inferred from their pellucid nature,—the smallest stars being distinctly visible through every part of them.
The envelope that surrounds the nucleus, varies in depth considerably, and appears but in few instances to unite with the nucleus : the depth of the envelope of the comet of 1811 amounted, when at its maximum, to no less than 25,000 miles, and its distance from the centre of the nucleus 30,000 miles ; the depth of the envelope of the comet of 1799 was estimated at nearly the same quantity.
The apparent lengths of the tails of comets vary from the smallest possible dimensions to that of the comet of 1618 and 1680, the former of which was more than a 100° in length; it must, however, be observed, that the tail of a comet will, at the same time, at different places, appear of different lengths, according to the states of the atmosphere at the different places of observation : there will also be a difference in the eyes of spectators at the same place; hence the disagreement between the estimates of astronomers :-at the time that the tail of the comet of 1680 was said to be 90°, as observed at London, it was stated to be 120° at Constantinople. It may be concluded that when the tail of a comet appears of considerable length, that it is then not very remote from the earth.
Comets may appear to be destitute of tails from their positions, relative to the earth and sun; if a comet approaches the sun in the direction of the earth, the axis of the tail is then coincident with the line of vision, consequently it appears with only a coma surrounding the nucleus without any divergency, and in every case where the line of vision does not pass, so as to form a right angle with the axis of the tail, it must appear shorter than it really is.
As the apparent lengths of these streams of light are in some instances very great, so are also their true lengths :--that of the comet of 1744 was calculated to be 20 millions of miles in extent, and that of 1769, a tail of 40 millions; the tail of the great comet of 1650 was computed to be no less than 100 millions of miles, and the second comet of 1811 had a vast projecting luminosity extending 132 millions of milės,--a length so vast as almost to overpower the faculties with astonishment,---for supposing the nucleus of this comet to be
placed on the orb of the sun, and its tail in the plane of the orbits of the planets-it would traverse across the orbits of Mercury, Venus, the Earth, and terminate only within 12 millions of miles of the orbit of Mars !
Behold! amidst yon wilderness of stars,
But away such thoughts,
Stars teach, as well as shine. At Nature's birth
OF THE SUN'S RISING AND SETTING, RIGHT ASCENSION,
DECLINATION, AND EQUATION OF TIME.
Each flower the dews have lightly wet,
Byron. SOLAR PHENOMENA. The Solar Spectrum. Had two observers, one situated in Mercury, and the other in Jupiter, studied the prismatic spectrum of the sun by the same instruments, and with the same sagacity as Newton, it is demonstrable, that they would have obtained very different results. On account of the apparent magnitude of the sun in Mercury, the observer there would obtain a spectrum entirely without green, having red, orange, and yellow, at one end, the white in the middle, and terminated at the other end with blue and violet. The observer in Jupiter would, on the contrary, have obtained a spectrum in which the colours were much more condensed. On the planet Saturn, a spectrum exactly