earth than the moon, as the earth is in bulk matter

greater than the moon.

The earth is judged to con

tain 40 times as much matter as the moon; there

fore, the centre of gravity is 40 times nearer the earth than the moon.

Any body, moving with a velocity round one centre, acquires a forcible tendency to go off from that centre, which is called the centrifugal force, and the attraction or power that keeps it towards the centre, is called the centripetal force. As an exemplification, let a person whirl round a ball at the end of a string a yard or two long, with his hand, he will be sensible, if he let the cord go, that the ball, or a planetary body, would fly off from the actions of the centrifugal force; but if he held it firm, the power of his hand resembling the attractive or centripetal force, would keep the body circulating in its orbit. It is to this cause, that philosophers now attribute the rise of the tides at the opposite parts of the earth to where the moon may be at the same time; for that part of the earth, which is opposite to the moon, exacts a greater centrifugal force from its centre, than that which is nearest to the centre of gravity, or facing that and the moon; and, therefore, as that part rises by the centrifugal force, and those opposite the moon by the centripetal force, or moon's attraction, as before mentioned.

The time of high water, at any place, is not when the moon is, in the meridian, but is generally about three hours afterwards; the cause given for which is as follows: The moon, when in the meridian, or

1

1

nearest the zenith of any place, tends to raise the waters at that place; and this force must evidently be exerted for some time, before the greatest elevation will take place; for, were the moon's attraction to cease altogether, when she has passed the meridian, yet the motions already communicated to the waters, would cause them to ascend for some time afterwards; and, therefore, they must be much more disposed to ascend, when the attractive force is only in a small measure diminutive.

Thus much may suffice to give the young student a general idea of the nature and rationale of the tides. They are subject to much deviation from the general rule given, on account of the various disproportions of the parts of the earth, and other local circumstances. To have a fuller and more comprehensive view of the subject, the reader must consult other larger and modern treatises on astronomy and navigation, where complete tables and demonstrations are given.

ESSAY II.

PART I.

A TREATISE

ON THE USE OF THE

TERRESTRIAL AND CELESTIAL GLOBES.

OF THE ADVANTAGES OF GLOBES IN GENERAL, FOR ILLUSTRATING THE PRIMARY PRINCIPLES OF ASTRONOMY AND GEOGRAPHY; AND PARTICULARLY OF THE ADVANTAGES OF THE GLOBES, WHEN MOUNTED IN MY FATHER'S MANNER.

UNIVERSAL approbation, the opinion of those

that excel in science, and the experience of those that are learning, all concur to prove that the artificial representations of the earth and heavens, on the terrestrial and celestial globes, are the instruments the best adapted to convey natural and genuine ideas of astronomy and geography to young minds.

This superiority they derive, principally, from their form and figure, which communicates a more just idea, and gives a more adequate representation of the earth and heavens, than can be formed from any other figure.

To understand the nature of the projection of either sphere in plano, requires more knowledge of geometry, than is generally possessed by beginners; its principles are more recluse, and the solution of problems more obscure*.

The motion of the earth upon its axis, is one of the most important principles both in geometry and astronomy; on it the greater part of the phenomena of the visible world depends; but there is no invention that can communicate so natural a representation of this motion, as that of a terrestrial globe about its axis. By a celestial globe, the apparent motion of the heavens, is always represented in a natural and satisfactory manner.

In order to convey a clear idea of the various divisions of the earth, of the situation of different places, and to obtain an easy solution of the various problems in geography, it is necessary to conceive many imaginary circles delineated on its surface, and to understand their relation to each other. Now, on a globe, these circles have their true form; their intersections, and relative positions, are visible upon the most cursory inspection. But, in projections of the sphere in plano, the form of these circles is varied, and their nature changed; they are, consequently, but ill adapted to convey, to young, minds, the elementary principles of geography.

On a globe, the appearance of the land and water

*See Keith's Introduction to Plane and Spherical Trigonometry, Sro. 1810.-EDIT.

is perfectly natural and continuous, fitted to convey accurate ideas, and leave permanent impressions on the most tender minds; whereas, in planispheres, one-half of the globe is separated and disjoined from the other; and those parts, which are contiguous on a globe, are here separated and thrown at a distance from each other. The celestial globe has the same superiority over projections of the heavens in plano.

The globe exhibits every thing in true proportion, both of figure and size; while, on a planisphere, the reverse may often be observed.

Presuming that these reasons sufficiently evince the great advantage of globes over either planispheres or maps, for obtaining the first principles of astronomical and geographical knowledge, I proceed to point out the pre-eminence of globes, mounted in my father's manner, over the common, or rather the old and Ptolemaic mode of fitting them up.

The great and increasing sale of his globes, mounted in the best manner, may be looked upon, at least as a proof of approbation, from numbers; to this, I might also add, the encouragement they have received from the principal tutors of both our universities, the public sanction of the university of Leyden, the many editions of my father's treatises on their use, and its translation into Dutch, &c. The recommendation of Messrs. Arden, Walker, Burton, &c. public lecturers in natural philosophy, might also be adduced; but, leaving these considerations, I shall proceed to enumerate the reasons which give