By Prof. C. W. MACCORD, Sc.D.
The construction of apparatus for illustrating the motions of the
heavenly bodies has often occupied the attention of both mathematicians
and mechanicians, who have produced many very ingenious, and in some
cases very complicated, combinations. These may be divided into two
classes; the object of the first being to represent _exactly_ what
occurs--to reproduce the precise movements of the various bodies
represented in their true proportions and relations to each other, in
respect to distances, magnitudes, times, and phases. When the absolute
complexity of the movements of the bodies composing the solar system
is considered, it is not so much a matter of wonder that a planetarium
which shall thus imitate them is a very delicate and complicated machine
as that it should lie within the limits of human ingenuity.
In the second class, the object is to show the nature and the causes
of specific phenomena, without regard to others perhaps, and without
necessarily paying attention to exact proportions of distances and
dimensions. Indeed, it is often the case that the illustration is made
clearer by exaggerating some of these and reducing others; thus, for
example, the causes of the variation in the lengths of the days and
nights, and of the changes in the seasons, can be exhibited to much
better advantage by an apparatus in which the diameter of the sun and
its distance from the earth are enormously reduced than they possibly
could be were they of their proper proportionate magnitudes; nor is the
presence of any other planet, or the attendance of a satellite, at all
necessary or even desirable for the purpose named.
It is apparent that machines of this class can be made much more simple
than those of the first, while at the same time it may safely be
asserted that for educational purposes they are far more useful.
In both classes, the action involves the use of some sort of epicyclic
train, since the motions to be explained are both orbital and axial. The
planetary body is carried round by a train-arm, and its rotation about
its axis is usually given it by a train of gearing, the inner or
central wheel of which is stationary, being fastened to the fixed frame
supporting the whole.
[Illustration: AN IMPROVED LUNARIAN.]
The lunarian which we herewith present belongs to the second of the
classes above named; in its construction an attempt has been made to
show by as simple means and in as clear a manner as possible the nature
of the following phenomena, viz.: