places is reckoned on this circle; and on the celestial, the right ascension of the stars, &c. it familiarizes the young pupil with them, and their reduction to time. This method does not, in the least, impede the motion of the globe; but, while it affords an equal facility of elevating either the north or south pole, it prevents the pupil from placing them in a wrong position; while the horary wire secures the globe from falling out of the frame.

Another circumstance peculiar to these globes, is the mode of fixing the compass. It is. self-evident, that the tutor, who is willing to give correct ideas to his pupil, should always make him keep the globes with the north pole directed towards the north pole of the heavens, and that, both in the solution of problems, and the explanation of phenomena. By means of the compass, the terrestrial globe is made to supply the purpose of a tellurian, when such an 'instrument is not at hand. I cannot terminate this paragraph, without testifying my disapprobation of a mode adopted by some, of making the globe turn round upon a pin in the pillar on which it is supported; a mode that, while, it can give little relief to indolence, is less firm in its construction, and tends to introduce much confusion in the mind of the pupil.

In order to prevent that confusion and perplexity, which necessarily arises in a young mind, when names are made use of which do not properly cha, racterize the subject, my father found it necessary, with Mr. Hutchins, to term that broad wooden cir

cle, which supports the globe, and on which the signs of the ecliptic, and the days of the month, are engraved, the broad paper circle, instead of horizon, by which it had been heretofore denominated. The propriety of this change, will be evident to all those who consider, that this circle, in some cases, represents that which divides light from darkness; in others, the horizon, and, sometimes, the ecliptic. For similar reasons, he was induced to call the brazen circle, in which the globes are suspended, the strong brass circle.

In a word, many operations may be performed by these globes, which cannot be solved by those mounted in the common manner; while all that they can solve may be performed by these, and that with a greater degree of perspicuity; and many problems may be performed by these, at one view, which, on the other globes, require successive operations.

But as, notwithstanding their superiority, the difference in price may make some persons prefer the old construction, it may be proper to inform them, that they may have my father's globes mounted in the old manner, at the usual prices*.

* Our late author's recommendation and arguments herein given, in favour of his father's globes, cannot now be fully admitted. There always have been some few inconveniences attending his mole of mounting, which is unnecessary to point out here. Globes being machines of illustration and exemplification only, and not designed for accurate calculation, I have always adopted and advised that mode of construction, which is the most simple and perspicuous to the beginner, in the solving of the problems;

I therefore think it very proper to acquaint the reader, that there were lately completed, new sets of plates for 18 and 12-inch globes, under the title of the NEW BRITISH GLOBES. It is now more than 40 years since the plates for the 17-inch of Senex's, and the 18-inch of Adams's, were engraved; and the important discoveries, both geographical and astronomical, that have recently been made, have afforded the opportunity of contributing to render the above new globes the most complete of any extant. On the terrestrial are inserted all the latest communications and places, from authentic observations, and engraved from a drawing by Mr. Arrowsmith; and, on the celestial, are placed near 6000 stars, clusters, planetary nebulæ, &c. from the latest observations of Dr. Maskelyne, Dr. Herschel, &c. as computed by me for the year 1801, so that these globes may be considered as properly adapted for the present century, and to supersede all former globes... EDIT.

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ESSAY II.

PART II.

CONTAINING

A DESCRIPTION OF THE GLOBES

MOUNTED IN THE BEST MANNER;

TOGETHER WITH SOME PRELIMINARY DEFINITIONS

DEFINITIONS.

BEFORE

we begin to describe the globes, it will be proper to take some notice of the properties of a circle, of which a globe may be said to be constituted. A line is generated by the motion of a point.

Let there be supposed two points, the one moveable, the other fixed.

If the moveable point be made to move directly towards the fixed point, it will generate in its motion a straight line.

If a moveable point be carried round a fixed point, keeping always the same distance from it, it will generate a circle, or some part of a circle, and the fixed point will be the centre of that circle.

All straight lines, going from the centre of the circumference of a circle, are equal.

Every straight line that passes through the centre of a globe, and is terminated at both ends by its surface, is called a diameter.

The extremities of a diameter are its poles.

If the circumference of a semicircle be turned round its diameter, as on an axis, it will generate a globe, or sphere.

The centre of the semicircle will be the centre of the globe; and as all points of the generating semicircle are at an equal distance from its centre, so all the points of the surface of the generated sphere are at an equal distance from its centre.

DESCRIPTION OF THE GLOBES.

There are two artificial globes. On the surface of one of them the heavens are delineated; this is called the celestial globe. The other, on which the surface of the earth is described, is called the terrestrial globe.

Fig. 2, plate 16, represents the celestial, fig. 1, plate 16, the terrestrial globe, as mounted in my father's manner.

In using the celestial globe, we are to consider ourselves as at the centre.

In using the terrestrial globe, we are to suppose ourselves on some point of its surface.

The motion of the terrestrial globe represents the real motion of the earth.

The motion of the celestial globe represents the apparent motion of the heavens.