But the sunbeam clung like a tender hand And clinging fast in gloom and blast, The glory grew and grew, Till the gaunt tree flashed in a robe of gold, And a glad thought brightened the weary face From the sunbeam's lesson a doubting heart Drew courage and hope again. E. A. B. COMETS.-But there are comets and comets, and it may be urged that we can not conclude they are all alike small and gravitationally powerless. Lexell's, however, was, to say the least, a fair sample. When it came nearest to us the measured diameter of its sphere of nebulosity (for it had no tail) was 59,000 miles, or five times the size of the moon. Its nucleus, which was very bright, had a tenth of this diameter, or nearly 6000 miles. The memorable comet of 1858, known as Donati's, vast and brilliant as was its vaporous surrounding, was corporeally smaller than Lexell's. Its solid (?) portion-its nucleuswas measured, and found to be at most only 500 miles in diameter, or about one-sixteenth that of the earth. Its volume would thus comprise sixtyfive millions of cubic miles of matter, about oneeightieth of the volume of the moon; and if the comet was not composed of denser or heavier matter than our satellite, its mass or weight would be one-eightieth of the moon's, and its gravitational effect, at the same distance, as small in proportion. Had either this comet or Lexell's come as close to us as the moon it would scarcely have exercised any appreciable influence on the tides or any other phenomenon or condition which can be affected merely by the mass or gravitational power of a proximate body. Certainly the comet in either case could not have made us its prisoner and carried us away into infinite space, or led us inwards to make fuel for the sun, or to be cindered by close contiguity to the luminary; and this was of old one of the dreaded consequences of a cometary approach. But may not a comet itself be such a fiery furnace as to affect us scorchingly, if it should but pass near us? We are hardly prepared to answer the question, in the present state of our knowledge. If only a good comet would make its appearance, no doubt some information would be speedily acquired concerning its thermal conditions, for in recent years an instrument has been used for measuring the radiant heat of the moon and stars, which no one had thought of applying when last a bearded star visited us. We allude to the thermo-electric pile, the thermometer, for such it is, so wonderfully sensitive that it will detect differences of temperature amounting only to a few millionths of a Fahrenheit degree. If another Donati would but exhibit itself we should doubtless soon have grounds for fairly judging whether a comet be an accumulation of hot combusting matter, or merely a body of cool substance glowing by some such property as phosphorescence. This, however, we have learnt within the past four years, thanks to the revelations of the spectroscope, that the light of several small comets which have appeared within this period has been identical with that emitted by the highly heated vapor of carbon. This shows cometary matter, so far, to be largely carbonaceous. But how comes the carbon into a state of apparently hot vapor? Some comets, it is true, have been known to approach the sun sufficiently near to acquire the fervent heat requisite to vaporize carbon; but this could hardly have been the case with the comets in question. The difficulty is removed if we assume that the carbon exists in combination with some decomposing element, such as oxygen or hydrogen; in this condition it is supposable that a moderate amount of solar heat would set up a combustion and satisfy the observed conditions. In the observations by Dr. Huggins, which revealed this carbon-vapor source of cometary light, the actual identity was established between it and the light of an electric spark passing through olefiant gas. It is open to conjecture whether electricity is in any way concerned in producing the light in the case of the comet.-The Gentleman's Magazine. |