7 o 4 THE POPULAR SCIENCE MONTHLY. 



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 born ? 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 conject- 

 ure that hydrogen is a more recent production than the worlds them- 

 selves ? 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 out- 

 bursts 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 UNITED STATES NAVAL OBSEKVATOKY. 



By EMMA M. CONVEESE. 



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, at- 

 tracted 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 dis- 

 turbed 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 com- 

 mittee. This committee strongly advocated in its report the erection 



