Page:Popular Science Monthly Volume 4.djvu/724

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as the spectroscope gives any indications of their constitution, it shows them to be composed of gases unknown in the earth.

As we have stated, the four outer planets are very nearly of the specific gravity of water; then come the innumerable asteroids, filling the place of a missing planet, and of which we know but little; then three planets that are five and a half times as dense as water; and lastly, Mercury, over eight times as dense. Does not this increasing density of the planets, from the outer to the inner, imply that they have become successively formed on the exterior of one great parent globe, and received each its portion, in the main, of denser elements, as it was later bora? That this effect should appear somewhat in groups of the planets, is owing, probably, to the absence or excess of oxygen among their components.

But, if this is so, what shall we say of hydrogen, the lightest of all the gases, which seems to be most abundant the nearer to the centre of the system? To explain this notable exception, might we conjecture that hydrogen is a more recent production than the worlds themselves? It has been observed time and again to burst up from the nethermost regions of the sun with inconceivable force, as if it were the pent-up product of a volcano, and to throw up columns of its flaming gas, in one case 200,000 miles high. And these great outbursts of hydrogen are always the precursors of the dark, sunken spots in the photosphere. How came this almost imponderable ether to be imprisoned in the deep craters of the sun, if it is not a product that is constantly forming in the solar caldron?

But it is easier to ask questions than to answer them. And I will close, in the fear of having been already thought too free with the scientific imagination.



THE importance of establishing a first meridian for the United States at the seat of government, in connection with a National Observatory for the purpose of systematic scientific observation, attracted the attention of Congress as early as 1810. In 181 3 the report of the committee, to whom the matter had been intrusted, was read before the House by one of its prominent members. But such were the disturbed condition of the country, and the absorbing interest in its military affairs during the war with Great Britain, that the subject was not again revived till 1815, when the original memorial with the several reports, hitherto presented, and the letter of the Secretary of State, read before the House in 1813, were referred to a select committee. This committee strongly advocated in its report the erection