our latitude, but at times very near the beginning and end of its appearance; because, by bringing the comet's path on one part to the south point of the horizon, it will immediately appear in what part the comet ceases to be visible; and then bringing the other part of the path to the point, it will appear in what part it will again become visible.

After this manner may the problems relating to

any other comets be performed; and thus the paths of the several comets, which have hitherto been observed, may be severally delineated on the celestial globe, and their various phenomena in different latitudes be thereby shewn.

Note.-A much more remarkable comet than the one of 1807, and which I noticed at page 166 of this work, made its first appearance in our northern hemisphere at the end of August last, and has continued till the present time (25th Dec. 1811), traversing among the northern constellations, with superior brilliancy, extent of coma and duration, to all other comets observed in the last century. It is now nearly at its extreme diminution of appearance, or at the vanishing distance. It was only first observed about its recess from the perihelion, or the sun, and the following is the best summary of its paths and elements, that could be collected from various correct observations, on clear nights:

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, fig. 4, is a diagram of the paths from the above observations. It was last seen near the star Altair, in the Eagle.-EDIT.

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 Cicero the 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, fig. 1, represents the planetarium.*

* This instrument, with two other parts, the Tellurian and Lunarium, plate xii. fig. 1 and 2, comprises an Orrery, the most portable 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 only, the motions displayed in this, and much more so, when the periodical revolutions of the satellites are included. The box ABC, is supported on a brass pillar and stand not shewn in the figure; and the arms are to be taken off their common central arbor, when either of the other two parts the Tellurian and Lunarium is to be applied. The diameter of the brass box is about 10 inches, apd

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 ivory balls, having the hemisphere which is next the sun white, the other black, to exhibit their respective

the machine and apparatus packs into a neat portable mahogany case, 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 revolutions of the satellites of Jupiter about that planet; and from illustrating the Jovian system, is denominated "The Jovarium." The two form a pair of complete instruments either for the student or lecturer.

In a similar manner, a machine for exhibiting the periodical revolution of Saturn's satellites may be constructed, EDIT.