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This comet approached nearest the earth about the 20th October, and was then at a distance of about 112 millions of miles, with a tail extending in length about 12 degrees. Dr. Herschel estimated its body to be in diameter, about 428 miles, enveloped in a cometic atmosphere.
Plate 10, Jig. 4, is a diagram of the paths from the above observations. It was last seen near the star Altair, in the Eagle,—Edit. .
OF THE MOST IHt'UOVEO
PLANETARIUM, TELLURIAN, AND LUNARIUM.
I NOW proceed, in pursuance of my original plan, to describe one of the instruments contrived to facilitate the study of geography and astronomy. It will realize to the eye of the pupil many phenomena, and impress them strongly on his memory. The instrument here described may be considered as one of the best hitherto contrived for explaining the"celestial motions. The description of this will, with very few alterations, apply to most other instruments designed for the same purpose. The explanation of the instrument will also enable me to render some articles plainer, and to treat others more fully; while those who have not thoroughly comprehended what has been already said, may gain more perfect ideas of the subject.
It seems highly probable, that the ancients were not unacquainted with planetary machines, and that the same powers of genius which led them to contemplate and reason upon the motion of the heavenly bodies, induced them to realize their ideas, and form instruments for explaining them ; and we may fairly presume, that these were carried to no small
degree of perfection, when we consider that of one,
Archimedes was the maker, and Cicerothe encomiast.
The instrument now to be described was invented
by the celebrated Huygens, though since his time
it has been ascribed to almost as many inventors as
makers; each deviation in form, the mounting it in
this mode or the other, the addition of a zodiac, or
some such slight changes, have been deemed by
many of sufficient importance to give them a claim
to the title of inventors :—be it so. Let the friend
of science encourage every humble effort to improve
it; and let him bestow a name which, though it
may in some measure gratify vanity, yet incites to
labour, rather than by contempt, check the ardour,
or discourage the talents which, when called forth,
may be of the greatest service to society.
DESCRIPTION OF THE PLANETARIUM.
Plate 11, Jig. 1, represents the planetarium.*
* This instrument, with two other parts, the Tel1urian and Lu~, narium, plate xii. fig. 1 and 2, comprises an Orrery, the most port. able and complete of any other made. By the triple division of the machine, the motions are more completely and accurately shewn than by the larger and usual kinds, where all the planetary motions are attempted to be exhibited in one machine. It requires a considerable bulk, as well as expence, to unite in one machine »nly, the motions displayed in this, and much more so, when th» periodical revolutions of the satellites arejilcluded. The box ABC, is supported on a brass pillar and stand not shewn in the"figure; and the arms are to be taken oft'their common central arbor, when either of the other two parts the Tellurian and Lunarium is to ba applied. The diameter of the brasi box is about 10 inches, and
The box ABC contains the wheel-work by which the planets are made to move round a brass ball S, representing the sun: this motion is communicated to them by turning the handle.
A planetarium may be considered, in some sort, as a diametrical section of our universe, in which the upper and lower hemisphere are suppressed.
The upper plate is to answer for the ecliptic; on this, therefore, are placed, in two opposite circles, corresponding to each other, the signs of the ecliptic, and the days of the month, by means whereof the planets may be easily set to their mean places in the ecliptic for any day in the year. Through the centre of the plate there passes a strong stem, on which the brass ball S is placed, which represents the sun; round the stem are the different sockets, which carry the arms, by which the balls representing the planets are supported. The planets are ivoiy balls, having the hemisphere which is next the sun white, the other black, to exhibit their respective places to each other. The planets may be easily put on or taken off their sockets, as occasion requires. About the primary planets are placed the secondary planets, or moons, which are in this instrument only moveable by hand; but when the instrument is fitted upon a large scale, and in a more expensive form, even thtvse are put in motion by th« wheel-work.
the machine and apparatus packs irdo a neat portable mahoganycase, about 20 inches in height, and 12 inches square.
This machine, with the exception of a few late improvements, was originally constructed by the late learned Optician, Mr. Benjamin Martin.
A similar machine, of correspondent figure and dimensions, is sometimes made to exhibit, by wheel-work, the periodical re, volutions of the satellites of Jupiter about that planet; and front illustrating the Jovian system, is denominated "The Jovarium." The two form a pair of complete instruments either for the student or lecturer. 'r
In a similar manner, a machine for exhibiting the periodical rerolution of Saturn's satellite! way be coBitructod,
The planets are disposed in the following order: in the centre is the brass ball 0, to represent the sun; then Mercury £ , Venus 9 , the earth ®, Mare £, Jupiter If., Saturn T^; then the Georgium Si
When the pupil has been gratified by putting the instrument in motion, and making his own observations on those motions, it will be proper to acquaint him with the names of the different planets, and of their division into primary and secondary, to shew him how they were first distinguished from the fixed stars, and how the length of their periodic revolution was discovered. Here it will be proper to observe, that the annual motion of the earth, or the time it takes to perform its period round the sun, is made the basis to which the others are compared; and this is one of the reasons why the months, and days of our months, are engraved on the circle. Having observed this, the planets may be put in motion, and they will be found to revolve round the representative of the sun in their proportionable times, each planet always completing its revolution in the same space of time, in periods regulated and pro