If we suppose a meteoric stone of the size of a six-inch cube to enter our atmosphere at the rate of eighteen miles per second of time, the atmosphere being 1/100 of its density at the earth’s surface, the resistance offered to the motion of the stone will in this case be at least 51,600 lbs.; and if the stone traverse twenty miles with this amount of resistance, sufficient heat will thereby be developed to give 1° Fahrenheit to 6,967,980 lbs. of water. Of course by far the largest portion of this heat will be given to the displaced air, every particle of which will sustain the shock, whilst only the surface of the stone will be in violent collision with the atmosphere. Hence the stone may be considered as placed in a blast of intensely heated air, the heat being communicated from the surface to the centre by conduction. Only a small portion of the heat evolved will therefore be received by the stone; but if we estimate it at only 1/100 it will still be equal to 1° Fahrenheit per 69,679 lbs. of water, a quantity quite equal to the melting and dissipation of any materials of which it may be composed.—Mr. J. P. Joule, On Shooting Stars: Phil. Mag. No. 216, p. 348.
[14] “Laplace conjectures that in the original condition of the solar system, the sun revolved upon his axis, surrounded by an atmosphere which, in virtue of an excessive heat, extended far beyond the orbits of all the planets, the planets as yet having no existence. The heat gradually diminished, and as the solar atmosphere contracted by cooling, the rapidity of its rotation increased by the laws of rotatory motion; and an exterior zone of vapour was detached from the rest, the central attraction being no longer able to overcome the increased centrifugal force. This zone of vapour might in some cases retain its form, as we see it in Saturn’s ring; but more usually the ring of vapour would break into several masses, and these would generally coalesce into one mass, which would revolve about the sun,”—Whewell’s Bridgewater Treatise.
The following passage is translated by the same author from Laplace:—
“The anterior state (a state of cloudy brightness) was itself preceded by other states, in which the nebulous matter was more and more diffuse, the nucleus being less and less luminous. We arrive in this manner at a nebulosity so diffuse, that its existence could scarce be suspected. Such is in fact the first state of the nebula which Herschel carefully observed by means of his telescope.”
Sir William Herschel has the following observations on these remarkable masses:—
“The nature of planetary nebulæ, which has hitherto been involved in much darkness, may now be explained with some degree of satisfaction, since the uniform and very considerable brightness of their apparent disc accords remarkably well with a much condensed, luminous fluid; whereas, to suppose them to consist of clustering stars will not so completely account for the milkiness or soft tint of their light, to produce which it would be required that the condensation of the stars should be carried to an almost inconceivable degree of accumulation.
“How far the light that is perpetually emitted from millions of suns may be concerned in this shining fluid, it might be presumptuous to attempt to determine; but notwithstanding the inconceivable subtilty of the particles of light, when the number of the emitting bodies is almost infinitely great, and the time of the continual emission indefinitely long, the quantity of emitted particles may well become adequate to the constitution of a shining fluid or luminous matter, provided a cause can be found that may retain them from flying off, or reunite them.”—Observations on Nebulous Stars: Philosophical Transactions, vol. lxxxi. a.d. 1791.
In addition, the following Memoirs on the same subject, by Sir William Herschel, have been published in the Philosophical Transactions:—Catalogue of 1000 Nebulæ and Clusters of Stars, vol. lxxvi.; Catalogue of another 1000, with remarks on the Heavens, vol. lxxix.; Catalogue of 500 more, with remarks as above, vol. xcii.; Of such as have a cometary appearance, vol. ci.; Of planetary nebulæ, ibid.; Of stellar nebulæ, ibid.; On the sidereal part of the heavens, and its connection with the nebulous, vol. civ.; On the relative distances of clusters of nebulous stars, vol. cviii.
[15] Lord Rosse’s beautiful telescopes have been formed upon principles which appear to embrace the best possible conditions for obtaining a reflecting surface which should reflect the greatest quantity of light, and retain that property little diminished for a length of time. The alloy used for this purpose consists of tin and copper in atomic proportions, namely, one atom of tin to four atoms of copper, or by weight 58·9 to 126·4.—On the Construction of large Reflecting Telescopes: by Lord Rosse. Report of the Fourteenth Meeting of the British Association, 1844, p. 79.
[16] The best description of the Zodiacal Light occurs in a letter furnished by Sir John Herschel to the Times newspaper in March, 1843:—“The zodiacal light, as its name imports, invariably appears in the zodiac, or, to speak more precisely, in the plane of the sun’s equator, which is 7° inclined to the zodiac, and which plane, seen from the sun, intersects the ecliptic in longitude 78° and 258°, or so much in advance of the equinoctial points: in consequence it is seen to the best advantage at, or a little after, the equinoxes; after sunset, at the spring, and before sunrise, at the autumnal equinox; not only because the direction of its apparent axis lies at those times more nearly perpendicular to the horizon, but also because at those epochs we are approaching the situation when it is seen most completely in section.