126 



SCIENCE. 



good service. Although he may not find any immediate 

 encouragement in the discovery of remarkable relations 

 among these stars, or the probable cause of their varia- 

 bility, he will be collecting observations that must form the 

 test of every theory. As examples of the result of intelli- 

 gent and persevering observation, we have the case of the 

 sun spots, which led directly to the discovery of their pe- 

 riod and its singular variability ; and that of the shooting 

 stars, which 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, with 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 ex- 

 tended, 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 may 

 be able to determine the motion of the solar system in 

 space, as well as the constant in precession. The constant 

 of aberration also needs a new determination ; and since 

 this constant is so closely connected 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 cin be 

 determined so easily and by other methods. The old 

 method of finding the value of this constant from the 

 eclipses of Jupiter's satellites may yet give us a trust- 

 worthy 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 se- 

 ries 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 investi- 

 gation, but in most cases the labor required is very great, 

 and the completion of the work would occupy a long time. 

 This follows of course from the fact that, with the refine- 

 ment of observations and their exact reduction, many small 

 terms must be considered which formerly could be ne- 

 glected. 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 Delaunay; 

 shall undertake its complete revision. This question should 

 now be looked on as a purely scientific one, and its definite 

 solution should be undertaken. The theory should not be 

 patched up by guesswork to fit the observations, but should 

 be carried out with the utmost rigor. 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 theo- 

 ries need a new investigation, and some of them are already 

 in the hands of able astronomers. That of Mercury is es- 

 pecially interesting in connection with the intra-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 theory, 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 aberration. These satellites 

 form a peculiar and interesting system, and their theory is 

 so complicated that the labor of correcting their elements 

 and forming new tables would be great, but still within the 

 power of a persevering astronomer. The recent discovery 



of the connection of comets with streams of meteors has 

 given additional interest to cometary astronomy, and there 

 is plenty of hard work to be done in reducing observations, 

 in computing perturbations, and in deducing the best or- 

 bits of the comets. The periodical comets have another 

 interest, since they mav give us information concerning the 

 matter filling space. It seems to be probable from different 

 reasons, such as the consideration of the light of the stars, 

 that there must be matter spread throughout the celestial 

 spaces ; but the only heavenly body that has directly give* 

 us information on this subject is Encke's comet, which has 

 a' period of 2 J A years. For a long time the motion of 

 this comet was very completely 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 

 theory 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 perturbations of the elements followed by Encke is 

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

 would be well to apply various methods. Here again, on 

 account of the frequent returns of the comet, the labor of 

 computation is very great, and probably would be enough 

 fully to occupy the time of one astronomer. The interest- 

 ing questions connected with the motion of this comet 

 ought to induce some one to undertake this laborious 

 work, 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 estab- 

 lished, that the future advancement of the science is as- 

 sured, especially since long intervals of time give an 

 increased value to observations. Yet we may hope lor 

 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 astronomer, 

 and they should understand each other better than is gen- 

 erally 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 illumina- 

 tion 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 cer- 

 tain 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 in- 

 strument, and have correct ideas of the ends 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 selection of sites for new observatories. It is 

 possible, perhaps probable, that our instruments may be 

 greatly enlarged and improved, and that important dis- 

 coveries 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 doing scientific work, or with much 

 thought or care for their condition ; but generally they are 

 built in connection with some college or academy, and are 

 the product of local and temporary enthusiasm, which 

 builds an observatory, equips it with instruments, and then 

 leaves it helpless. The atmosphere that surrounds 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 the 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 way wc may be able to do 



