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 can 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 given 
us information on this subject is Encke’s comet, which has 
a’ period of 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 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 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 we may be able to do 
