90 



SCIENCE. 



[Vol. XII. No. 290 



of Hansen. These, like Leverrier's tables of the inner planets, 

 are now more than thirty years old. These tables have been com- 

 pared with observations, and agree fairly well with those made 

 during the century preceding their publication, but not with those 

 made before or since that time. The theoretical value of the accel- 

 eration of the moon's longitude is 6" ; that found by Hansen from 

 accounts of ancient total eclipses of the sun, 12". Newcomb, how- 

 ever, considers these accounts as unreliable, and, limiting himself 

 to the Ptolemaic eclipses of the Almagest and the Arabian eclipses of 

 th Table Hakemite, obtains the value 8".3, or, from the Arabian 

 eclipses alone, 7", — a value but little greater than the theo- 

 retical value. Dr. Ginzel, from an extended examination of 

 accounts of ancient and mediseval total eclipses of the sun, con- 

 cludes that Hansen's value requires a change of only a little over 

 l". His solution, however, in reality depends upon the ancient 

 eclipses alone. The only other theory of the moon comparable 

 with Hansen's is that of Delaunay. This theory, however, is 

 limited to a determination of the inequalities in the motion of the 

 moon due to the action of the sun, on the hypothesis that the orbit 

 of the earth is a pure ellipse, and differs from that of Hansen in 

 that the inequalities determined are not expressed numerically, but 

 only symbolically in terms of arbitrary constants. 



While the co-efficients of the inequalities upon which Hansen's 

 tables are based seem to be pretty well known, I am not aware 

 that the tables themselves have been sufficiently checked, except 

 by comparison with observations. Apparently the great desidera- 

 tum now is a set of tables computed from Delaunay 's theory in a 

 completed form, or computed in some other way entirely independ- 

 ently of Hansen's. Until Hansen's tables are thus checked, it is 

 questionable whether it can be safely said that the motion of the 

 moon cannot be completely accounted for by the law of gravity. 



The detection of the two satellites of Mars by Professor Hall 

 may be considered the most interesting recent achievement in pure 

 discovery. It was not till the discovery of these satellites that a 

 means was offered for the accurate determination of the mass of 

 that planet. No satellites of Venus and Mercury have as yet been 

 detected, and the values at present assumed for the masses of 

 those planets are very uncertain. 



In 1788, just one hundred years ago, Laplace published his 

 theory of Jupiter's satellites. This theory is still the basis of the 

 tables now in use. Souillart's analytical theory of these satellites 

 appeared in 1881. The numerical theory was completed only 

 within the last year, and the tables therefrom remain still to be 

 formed. 



Bessel made a careful investigation of the orbit of Titan ; but the 

 general theory of the Saturnian system which he commenced, he did 

 not live to finish. Our knowledge of the motions of Saturn's 

 satellites, with the exception of Titan, was very meagre until the 

 erection of the great equatorial at Washington. A difficulty in the 

 determination of a correct theory of the motions of Saturn's satel- 

 lites is the fact that there are a number of cases of approximate 

 commensurability in the ratios of their mean motions. The most 

 interesting case is that of Hyperion, whose mean motion is very 

 nearly three-fourths that of Titan. In this case there is the addi- 

 tional difficulty that their distance from one another is only about 

 one-seventh as great at conjunction as at opposition. 



Our knowledge of the motions of the satellites of Uranus and 

 Neptune depends almost entirely on the observations made at 

 Washington. Quite accurate determinations of the masses of these 

 two planets have been obtained. The large secular motion of the 

 plane of Neptune's satellite, to which Marth has called attention, 

 needs confirmation. 



The number of the asteroids is so great that they have been the 

 frequent subject of statistical investigation. The systematic group- 

 ing of the nodes and perihelia which exists was shown by Newcomb 

 to be the effect of perturbation. Glauser finds that the grouping 

 of the nodes on the ecliptic is a result of a nearly uniform distri- 

 bution on the orbit of Jupiter. Professor Newton had previously 

 found that the mean plane of the asteroid orbits lies nearer to the 

 plane of Jupiter's orbit than to the orbit plane of any individual 

 asteroid. Eighty-five per cent of the asteroids have mean motions 

 greater than twice and less than three times that of Jupiter; and 

 the mean motions of none approximate closely either of these, the 



two simplest ratios possible. The next simplest ratios lie beyond 

 the limits of the zone ; that is, there are no asteroids having mean 

 motions nearly equal to or less than one and a half times that of 

 Jupiter, and none nearly equal to or greater than four times that of 

 Jupiter. The labor of determining the general perturbations and 

 computing tables of an asteroid is as great as in the case of a major- 

 planet. It is no wonder, therefore, that tables have been prepared 

 for scarce a dozen of these small bodies, and that these are already 

 out of date. 



Of well-known comets of short period, Encke's, which has the 

 shortest period of any, possesses the greatest interest to the student 

 of celestial motions, since it was from a discussion of the orbit of 

 this comet that Encke detected evidence of the existence of a re- 

 sisting medium which produces an acceleration in the comet's- 

 mean motion. This acceleration has been confirmed by the inves- 

 tigations of Von Asten and Backlund. The investigations of 

 Oppolzer and Haerdll indicate that there is an acceleration also in 

 the mean motion of Winnecke's comet. 



We have thus glanced briefly at the present condition of our 

 knowledge of the motions of the principal bodies of the solar 

 system. Only four cases have been found in which we cannot 

 fully explain these motions, so far as known, by Newton's law of 

 gravity. The unexplained discordances are the motion of the 

 perihelion of Mercury, and the accelerations of the mean motions of 

 the moon and the two periodic comets just named. 



If we go beyond the solar system, we cannot tell whether New- 

 ton's law does or does not apply without modification to all parts 

 of the universe. Itis principally in the hope of answering this ques- 

 tion that double-star observations are carried on; and, in the case 

 of the many binary systems already detected, Newton's law is 

 satisfied within the errors of observation. Nevertheless, this evi- 

 dence is purely negative, and its value, it seems to me, not at all 

 commensurate with the labor expended upon it, unless it be in the 

 case of such objects as Sirius, whose observation may assist in the 

 solution of the problem of irregular so-called proper motion. The 

 angles subtended are in general so small that relatively large per- 

 sonal errors are unavoidable; so that, even though their motions be 

 controlled by a law or laws of gravity widely different from that of 

 Newton, it is not likely that such differences can be proved with 

 any degree of certainty. It is rather to the study of the proper 

 motions of the fixed stars and of the nebulse, and then only after a 

 lapse of hundreds and perhaps thousands of years, that we must 

 look for a solution of this question. 



SOME PHASES IN THE PROGRESS OF CHEMISTRY.' 



Since the isolation of oxygen by Priestley, the search fornew ele- 

 ments has been carried on vigorously, and the facilities for this pur- 

 suit have been much increased by the use of the delicate spectro- 

 scopic methods. The result has been to continually extend the list 

 of bodies which are grouped under this head. The announcement 

 of new discoveries during the last ten years has been especially 

 large, over seventy bodies having been added to the list during this 

 time. The largest number added by any observer has resulted 

 from the joint labors of Kriiss and Nilson on the absorption spectra 

 of the rare earths, and reaches to over twenty. Should these discov- 

 eries be verified, the possible number of compounds which would 

 result is something enormous, but, judging from experience, few 

 are likely to survive a very searching inspection ; yet one of them,, 

 'germanium,' discovered by Winkler in 1886, has already been ac- 

 cepted as one of the missing elements in Mendelejeff's scheme, 

 whose existence and properties he predicted. 



Since the unit weight of hydrogen is taken as the standard for 

 comparison, while the determination of the atomic weights of a 

 large number of the elements has been made only through the 

 intervention of oxygen, the ratio of the atomic weights of these two- 

 elements is the most^important one to be determined, and many at- 

 tempts have been made to solve this problem. The older experi- 

 ments of Dumas and others were recently subjected to a careful 

 scrutiny, and it was shown that they were not sufficiently exact. As 



^ Abstract of an address before the Section of Chemistry of the American Associa- 

 tion for the Advancement of Science at Cleveland, O., Aug. 15-22, i883, by C. E. 

 Munroe, vice-president of the section. 



