Od. 14, 1880] 



NA TURE 



573 



the<e star^;, or the probable cau?e of their variability, he will be 

 coliecting observations that mnst form the test of every theory. 

 As exaiiiples of the result of intelligent and persevering observa- 

 tion, we have the case of the sun-spots, which led directly to 

 the 'discovery of their period, and its singular variability ; and 

 that of the shooting stars, \\-hich has shown us a very curious 

 relation between these meteors and the comets, and one which 

 may open to us the most extensive views of the relations between 

 our own solar system and other systems in space. 



The present condition of astronomy, w^ith its vast and rapidly 

 increasing store of accurate observations, offers many interesting 

 subjects to the theoretical astronomer. The observations of the 

 stars are now so numerous, and have been so fully reduced and 

 criticised, and the time during which the observations have been 

 made is so extended, that we shall soon have excellent data for 

 a new and very exact determination of the constant of precession. 

 The orbits of the planets and the moon, and their masses, are 

 now so well known, that little uncertainty can arise from this 

 source; and by taking into the calculation a great number of 

 stars in different parts of the heavens, we m.ay be able to 

 determine the motion of the solar system in space, as well as the 

 constant of precession, The constant of aberration also needs a 

 new determination, and since this constant is «o closely con- 

 nected with the theory of light and its velocity, and the methods 

 of its determination are still under discussion, it would be well if 

 several astronomers could determine this constant independently. 

 The value we now use was found by W. Struve from prime- 

 vertical observations, and is apparently very accurate ; but no 

 astronomical constant should depend on the work of a single 

 astronomer with a single instrument, when it can be determined 

 so easily and by other methods. The old method of finding the 

 value of tliis constant from the eclipses of Jupiter's satellites may 

 yet give us a trustworthy value. The value of the other constant 

 necessary for the reduction of observations, that of nutation, must 

 be nearly that found by Peters in his well-known investigation of 

 this question. This value may be verified by a new series of 

 observations of Polaris, or of the declinations of stars situated so 

 that this constant has its full influence on the reductions. 



There are many subjects in astronomy that need investigation, 

 but in most cases the labour required is very great, and the com- 

 pletion of the work would occupy a long time. This follows of 

 course from the fact • that, with the refinement of observations 

 and their exact reduction, many small terms must be considered 

 which formerly could be neglected. The lunar theory has been 

 a vexed question for the last two centuries, and may remain so 

 for a long time to come. This will no doubt be the case until 

 some able astronomer, with the will and perseverance of De- 

 launay, shall undertake its complete revision. This question 

 should now be looked on as a purely scientific one, and its de- 

 finite solution should be undertaken. The theory should not be 

 patched up by guesswork to fit the observations, but should be 

 carried out vi\\\\ the utmost rigour. This is a problem to which 

 a young and able mathematician may well devote his life, and we 

 must expect its solution from some such clear-headed devotee of 

 science. Several of the planetary theories need a new investi- 

 gation, and some of them are already in the hands of able 

 astronomers. That of mercury is especially interesting in con- 

 nection with the iutra-Mercurial planets, and it is to be hoped 

 that Leverrier's theory of this planet may soon have a careful 

 revision. 



Again, among the secondary systems, the satellites of Jupiter 

 and Saturn offer many interesting questions to the astronomer. 

 At present the satellites of Jupiter demand a more complete 

 theoi-y and new tables of their motions. Corrected elements of 

 these satellites may be required for reducing observations of their 

 eclipses, and for deriving a new value of the constant of aberra- 

 tion. These satellites form a peculiar and interesting system, 

 and their theory is so complicated that the labour of correcting 

 their elements and forming new tables would be great, but still 

 within the power of a persevering astronomer. The recent dis- 

 covery of the connection of comets with streams of meteors has 

 oiven additional interest to cometary astronomy, and there is 

 plenty of hard work to be done in reducing obser^'ations, in 

 computing perturbations, and in deducing the best orbits of the 

 comets. The periodical comets have another interest, since they 

 may give us information concerning the matter filling space. It 

 seems to be probable from different reasons, such as the consi- 

 deration of the light of the stars, that there must be matter 

 spread throughout the .celestial spaces ; but the only heavenly 

 body^that has directly given us information on this subject 



is Encke's comet, which has a period of three years and a 

 third. For a long time the motion of this comet was very com- 

 pletely computed by Encke, whose calculations show very strong 

 proof of a resisting medium. These calculations were continued 

 by Von Asten, whose early death prevented him from finishing 

 his work, and the theoiy of this comet is left in an unsatisfactory 

 condition. It is very desirable that the motion of this comet 

 should be completely investigated, and although the method of 

 the special pertm-bations of the elements f oUow ed by Encke is 

 probably the best that can be used, still in such a case it would 

 be w ell to apply various methods. Here again, on account of 

 the'frequent returns of the comet, the labour of computation is 

 vei-y great, and probably would be enough fully to occupy the 

 time of one astronomer. The interesting questions connected 

 with the motion of this comet ought to induce some one to under- 

 take this laborious %\ ork, and these questions are so important 

 that two or three astronomers might well be employed on its 

 theory. 



The methods of astronomy have now become so well esta- 

 blished that the future advancement of the science is assured, 

 especially since long intervals of time give an increased value to 

 observations. Yet we may hope for improvement in instruments, 

 for the introduction of new methods of observing, for better 

 trained and more efficient astronomers ; and perhaps also the 

 rapid advancement of the physical sciences may furnish us with 

 new and more powerful methods of investigation. There is an 

 intimate relation between the instrument-maker and the astro- 

 nomer, and they should understand each other better than is 

 generally the case. It may seem a small matter that the divi- 

 sions of a circle, or of a scale, should not be too finely or too 

 coarsely cut ; that the reading scale should not be placed in an 

 inconvenient position, and that the illumination of the instrument 

 should be carefully studied, and brought under the control of the 

 astronomer ; but these are really essential points, and, if not 

 rightly arranged, are certain to weary the observer and to impair 

 the quality of his work. Such mistakes will not be remedied 

 until the makers better understand the uses of an astronomical 

 instrument, and have correct ideas of the end to be attained. 

 Since our American opticians have placed themselves at the 

 head of their craft, we may hope that our instrument-makers 

 will do likewise, and that they will soon be able to furnish us 

 with the best instruments of precision. 



There is one point to which astronomers should give more 

 attention, and from which we may reasonably hope that great 

 advantages to astronomy may come ; and that is to the selectiott 

 of sites for new observatories. It is possible, perhaps probable, 

 that our instruments may be greatly enlarged and improved, and 

 that important discoveries and improvements in the manufacture 

 of optical glass may be made ; but it seems certain that we have 

 within easy reach very decided advantages for astronomical work 

 by the choice of better positions for our instruments. Very few 

 American observatories have been established for the purpose of 

 doin.^ scientific work, or with much thought or care for their 

 future condition ; but generally they are built in connection with 

 some college or academy, and are the product of local and tem- 

 porary enthusiasm, wdiich builds an observatory, equips it with 

 instruments, and then leaves it helpless. The atmosphere that sur- 

 rounds us, and its sudden changes of temperature, are the great 

 obstacles to the good performance of a telescope ; and the larger 

 the instrument, and the higher Uie magnifying power, the more 

 serious are these hindrances. Now, with our present means of 

 travel we can easily place our instruments at an altitude of eight 

 or ten' thousand feet, and above a large part of the atmosphere. 

 In this w.ay we may be able to do with small instruments what 

 at common altitudes can be done only with large ones ; and when 

 possible it is always better to use smaU instruments, since they 

 are more easily handled, and are relatively stronger and better 

 than large ones. Uniformity of temperature may be secured by 

 seeliing locations in the tropical islands, or on coasts like that of 

 California, where the ocean winds keep the temperature nearly 

 uniform throughout the year. At great altitudes we may secure 

 a clearness of vision that would be of the greatest value in the 

 examination of faint objects, and by this means, and by perse- 

 vering and continuous observation, interesting discoveries may 

 be made. It is a matter of course that, except in the case of 

 comets, the future discoveries in astronomy will belong to faint 

 and delicate objects ; but these are interesting, and should not be 

 ne<'lected A uniform temperature, which secures good defini- 

 tiou, and steady images of the stars, is necessary for accurate 

 determinations of position, and for aU measurements of pre- 



