The minuteness of those differences by which our terrestrial ellipsoid has been proved to deviate from a perfect globe, strikingly exhibits the accuracy and nicety of the methods employed by astronomers and mathematicians. What delicate instruments must have been used, what rigorous calculations made, in order to determine, within a few toises of the truth, the dimensions of this vast globe, in comparison of which our body is but an atom! Let not this discovery at least be attributed to the ancients! If some learned men have pretended to see clearly, in some Whether vague phrases of the ancients, a notion of the polar depression, there are others who have as clearly discovered acquainted the idea of an equatorial depression; those two opposite depression opinions, therefore, destroy each other. The notion even of of the the ellipticity of the terrestrial globe could not arise but globe. from a clear idea of universal gravitation. It was therefore reserved for the genius of modern geometry to draw the human mind into this bold and subtle research.

the an

cients were

with the

Freret, Mem. de l'Acad. des Inscriptions, tom. xviii. p. 112.

Burnet, Theoria Telluris Sacra, p. 26, 136, 137.

BOOK III.

Continuation of the Theory of Geography. Of Terrestrial
Globes. Of their Construction and principal Uses.*

To fix well in the mind the different parts of knowledge
which form the study of geography, it is necessary to have
before our eyes an image of the earth and its parts, on a
small scale. The most simple of these representations is
the artificial terrestrial globe; it is the earth in relief, with
its seas, continents, and islands. The mountains, rivers,
and principal towns, are also indicated on it. All these
points have on the artificial globe their true position; they
are represented in their totality, and relatively to each other,
as they are situate on the earth itself, according to astro-
nomical observations and geodesic measures.
A geogra
phical map can only give perspective views of a part of
the globe, in which there are always more or less of errors
of convention.

us tion of the the globe.

artificial

We find in the artificial globe the material image Descripof those mathematical circles which serve to give an idea of the various relations of the earth with heavenly bodies, and of terrestrial places with each other. Thus, on the surface of the globe ought to be indicated, the terrestrial equator, the tropics, the polar circles; then, by weaker lines, the other parallels to the equator, from 5 to 5, or from 10 to 10 degrees, according to the size of the globe. We also find the meridians indicated from 5 to 5 or from 10 to 10; they are numbered at their point of intersection with the equator. The parallels to the equator are also numbered at the place where they cut that meridian which has been chosen for the first. The ecliptic is also marked on good globes.

Bion, Usage des Globes, 1718. Schiebel, Instruction sur l'usages des Globes Artificiels (en Allemand) 1779 & 1785.

BOOK

III.

Rules to

choose a globe.

The poles are indicated by two points, on the axis of which the globe turns. These two points are fixed to a circle of metal which surrounds the globe from one pole to the other, so that on turning the globe, every terrestrial spot passes under this circle. It serves, therefore, as a general meridian, and is so called. The degrees of latitude, and even, on large globes, the minutes and seconds, are marked on the general meridian.

The bearers, or feet of the whole machine, support a circular band of metal or wood; it cuts the globe, in whatever position it may be placed, into two hemispheres, one superior, the other inferior; and thus represents the rational horizon. This artificial horizon has several circles traced on its surface; the inmost marks the number of degrees of the twelve signs of the zodiac; on it are the names of those signs and the days of the month. Another circle is divided into thirty-two parts, which mark the points of the compass.

The quadrant for taking heights is intended as a substitute for the compass in different researches. It is a little plate of copper attached to the general meridian, and divided into 90 degrees,* which serves to measure the distance and position of the places without the compass. The horary circle is fixed on the north pole; it is divided into 24 hours, and bears a moveable needle, which turns round the axis of the globe. There is also at the foot of the globe a compass, which should be fixed in the parallel and meridian of the horizon.

Globe-makers, and especially those of Paris, have been so careless of late years, in the delicate construction of that instrument, that a lover of geography cannot be too scrupulous in examining the quality of a globe before he purchases it. He should ascertain the complete correspondence of the divisions marked on the circles. The degrees. of the equator and the ecliptic should be equal with each other and with those of the quadrant. The same equality

*It commonly goes to 114 degrees, or to the arch equal to the diameter.

63

should exist between the degrees of the general meridian BOOK and the horizon, represented by the interior circle of the circular band of the middle. These divisions are examined, by intercepting with a compass a certain number of degrees, and by trying if, with the same opening of the compass, the same number of degrees can be intercepted every where. The globe should be at an equal distance from the general meridian and' from the horizon, and far enough. from them never to rub against those circles; which only happens in the very worst globes. The globe should be balanced perpendicularly on the two points which represent the poles. This is known, if, when turned, it stops as soon as the hand is taken from it. The equator should, in all positions, cut the meridian, and, if there be one, the horizon, into two equal arches; it ought therefore always, on turning with the globe, to coincide with the points where the quarter of those circles begin. In the parallel sphere, it should always preserve the most exact parallelism with the horizon. In like manner, the tropics and polar circles should every where coincide with the latitudes that belong to them.

The network, or assemblage of the lines representing the circles of longitude and latitude, should correspond exactly in all its joinings; which is very seldom the case even in large globes; the surface of the paper pasted on the globe being rarely connected with perfect exactness.

globe.

The globe serves, generally speaking, to recapitulate the Construcelements of mathematical geography. In order to show its tion of the use, we shall now explain its primitive construction. The most simple and most exact way of constructing a globe is to draw immediately on its surface, by the means we are about to describe, the circles, lines, and points, which it ought to represent.*

A

Let us suppose that two points diametrically opposite, have been fixed to represent the poles, and that the axis of rotation is to pass through them: taking one of these points

* Varenius, General Geography, b. iii. chap. 32. prop. 5.

III.

BOOK for a centre, at an equal distance from each, a circle must be described, which will be the equator; another great circle will be drawn through the poles to represent the first meridian, which will be divided into 90 degrees, setting out from the equator towards each pole: afterwards, setting out from this meridian, the circumference of the equator must be divided from degree to degree. These two circles being determined, it is easy to mark on the globe a place, the latitude and longitude of which may be learnt from the geographical tables; for it will be sufficient to mark the former on the first meridian, and through the point where it falls, must be described, taking the pole for the centre, the circle parallel to the equator, passing through the proposed spot; then, drawing through the point of the equator on which the longitude falls, and through the poles, a semicircle, the meridian will be had, whose junction with the parallel marks the position of the place.

It is thus that the circles of latitude and of longitude are traced on the globe, at the distance of 10 or 5 degrees from each other. With respect to these circles, the following remark may be perhaps a little too elementary for most of our readers.

Decrease of The circles of latitude are parallel to the equator; they the degrees diminish therefore necessarily till the last circle of latitude of paral

lels.

is identified with the point of the pole itself. The circles of longitude, or the meridians, go from pole to pole, and cut the equator perpendicularly; they are equal, with a very slight difference. The degrees of latitude are counted only on the circles of longitude, and vice versa. The degrees of latitude are, therefore, little arches of of a circle of longitude, intercepted by two circles of latitude. They would of course be equal without this small difference, which proceeds from the depression, and makes them increase a little towards the poles. The degrees of longitude are little arches of of a circle of latitude, intercepted by two circles of longitude. Therefore the degrees of longitude go on diminishing in proportion as the circles of longitude come near each other; and at the point where