LECT. VIII. & IX.

The ter

restrial globc.

Proof of

LECTURES VIII. AND IX.

THE DESCRIPTION AND USE OF THE GLOBES, AND
ARMILLARY SPHERE.

IF a map of the world be accurately delineated on a spherical ball, the surface thereof will represent the surface of the earth: for the highest hills are so inconsiderable with respect to the bulk of the earth, that they take off no more from its roundness, than grains of sand do from the roundness of a common globe; for the diameter of the earth is 8000 miles in round numbers, and no known hill upon it is three miles in perpendicular height."

That the earth is spherical, or round like a globe, the earth's appears, 1. From its casting a round shadow upon the being globular, moon, whatever side be turned towards her when she is eclipsed. 2. From its having been sailed round by several persons. 3. From our seeing the farther, the higher we stand. 4. From our seeing the masts of a ship, whilst the hull is hid by the convexity of the water."

And that

all sides

The attractive power of the earth draws all terresit may be trial bodies towards its center; as is evident from the peopled on descent of bodies in lines perpendicular to the earth's surface, at the places whereon they fall; even when they are thrown off from the earth on opposite sides, and consequently, in opposite directions. So that the

without any one

being in danger of falling

away from

it.

Note 71. The principal mountains of the earth have lately been delineated on a terrestrial globe, contrived by an Italian. It will, however, be apparent, from what our Author has stated above, that the proportions could not have been accurately represented.

Note 72. It may be proper to repeat what has been already stated, that the earth is not a perfect sphere, but an oblate spheroio: the poles being depressed. while the equator is expanded

earth may be compared to a great magnet, rolled in LECT. filings of steel, which attracts and keeps them equally VIIL& IX. fast to its surface on all sides. Hence, as all terrestrial bodies are attracted towards the earth's center, they can be in no danger of falling from any one side of the earth, more than another."

The heaven or sky surrounds the whole earth: and Up and down, when we speak of up or down, we mean only with re- what. gard to ourselves; for no point, either in the heaven or on the surface of the earth, is above or below, but only with respect to ourselves. And let us be upon what part of the earth we will, we stand with our feet towards its center, and our heads towards the sky and so we say, it is up towards the sky, and down towards the center of the earth.

:

re

in the heavens ap

and pear equally

To an observer placed any where in the indefinite All objects space, where there is nothing to limit his view, all mote objects appear equally distant from him; seem to be placed in a vast concave sphere, of which distant. his eye is the center. Every astronomer can demonstrate, that the moon is much nearer to us than the sun is; that some of the planets are sometimes nearer to us, and sometimes farther from us, than the sun; that others of them never come so near us as the sun always is; that the remotest planet in our system is beyond comparison nearer to us than any of the fixed stars are ; and that it is highly probable some stars are, in a manner, infinitely more distant from us than others. And yet all these celestial objects appear equally distant The face from us. Therefore, if we imagine a large hollow sphere of the of glass to have as many bright studs fixed to its in-heaven side as there are stars visible in the heavens, and these represent

Note 73. A reference to this fact will shew that two pendulums or plumb-lines suspended freely on the earth's surface, would not hang parallel to each other, but assume such a direction as would bring them in contact at the center of gravity of the globe.

and earth

ed in a machine.

LECT. studs to be of different magnitudes, and placed at the VIII. &IX.

same angular distance from each other as the stars are the sphere will be a true representation of the starry heaven, to an eye supposed to be in its center, and viewing it all around. And if a small globe, with a map of the earth upon it, be placed on an axis in the center of this starry sphere, and the sphere be made to turn round on this axis, it will represent the apparent motion of the heave: round the earth.

If a great circle be so drawn upon this sphere as to divide it into two equal parts, or hemispheres, and the

plane of the circle be perpendicular to the axis of the The equi. sphere, this circle will represent the equinoctial, which noctiul

divides the heavens into two equal parts, called the

northern and the southern hemispheres; and every point The poles. of that circle will be equally distant from the poles, or

.
ends of the axis in the sphere. That pole which is in
the middle of the northern hemisphere, will be called
the north pole of the sphere, and that which is in
the middle of the southern hemisphere, the south
pole.

If another great circle by, drawn upon the sphere, in in such a manner as to cut the equinoctial at an angle

of 23; degrees in two opposite points, it will represent The eclip- the ecliptic, or circle of the sun's apparent annual motion;

one half of which is on the north side of the equinoctial, and the other half on the south.

If a large stud be made to move eastward in this ecliptic in such a manner as to go quite round it, in the

time that the sphere is turned round westward 366 The sun. times upon its axis ; this stud will represent the sun,

changing his place every day a 365th part of the ecliptic; and going round westward, the same way as the stars do ; but with a motion so much slower than the motion of the stars, that they will make 366 revolutions about the axis of the sphere, in the time that the

tie.

.

The ap

siin makes only 365. During one half of these revo

LECT.

VIII.& IX. lutions, the sun will be on the north side of the equinoctial ; during the other half, on the south ; and at the end of each half, in the eqinoctional.

If we suppose the terrestrial globe in this machine to the earth be about one inch in diameter, and the diameter of the starry sphere to be about five or six feet, a small insect on the globe would see only a very little portion of its surface; but it would see one half of the starry sphere, the convexity of the globe hiding the other half from its view. If the sphere be turned westward round the globe, and the insect could judge of the appearances parent

motion of wbich arise from that motion, it would see some stars the hea. rising to its view in the eastern side of the sphere, whist vens. others were setting on the western : but as all the stars are fixed to the sphere, the same stars would always rise in the same points of view on the east side, and set in the same points of view on the west side. With the sun it would be otherwise, because the sun is not fixed to any point of the sphere, but moves slowly along an oblique circle in it. And if the insect should look towards the south, and call that point of the globe, where the equinoctial in the sphere seems to cut it on the left side, the east point: and where it cuts the globe on the right side, the west point : the little animal would see the sun rise north of the east, and set north of the west, for 182 revolutions ; after which, for as many more, the sun would rise south of the east, and set south of the west. And in the whole 365 revolutions, the sun would rise only twice in the east point, and set twice in the west. All these appearances would be the same, if the starry sphere stood still (the sun only moving in the ecliptic) and the earthly globe were turned round the axis of the sphere eastward. For, as the insect would be carried round with the globe, he would be quite sensible of its motion; and the sun and stars woula appear to move westward.

zon

LECT. We are but very small beings when compared with VIII. & IX.

our earthly globe, and the globe itself is but a dimension

less point compared with the maguitude of the starry The opera beavens. Whether the earth be at rest and the heaven glass.

turns round it, or the heaven be at rest, and the earth turns round, the appearance to us will be exactly the same. And because the heaven is so immensely large,

. in comparison of the earth, we see one half of the heaven as well from the earth's surface, as we could do from its center, if the limits of our view were not inter

cepted by hills. Circles of We may agine as many circles described upon the the sphere. earth as we please ; and we may imagine the plane of

any circle described upon the earth to be continued, until it marks a circle in the concave sphere of the heaven.

The horizon is either sensible or rational. The sensible The hori- horizon is that circle, which a man standing upon a

large place, observes to terminate his view all around where the heaven and earth seem to meet. The plane of our sensible horizon continued to the heaven, divides it into two hemispheres; one visible to us, the the other hid by the convexity of the earth.

The plane of the rational horizon, is supposed parallel to the plane of the sensible; to pass through the center of the earth, and to be continued through the heavens. And although the plane of the sensible horizon touches the earth in the place of the observer, yet this plane, and that of the rational horizon, will seem to coincide in the heaven, because the whole earth is but a point compared to the sphere of the heaven.

The earth being a spherical body, the horizon or

limit of our view must change as we change our place. Polas. The poles of the earth are those two points on its sur

face in which its axis terminates. The one is called the north pole, and the other the south pole.

The poles of the heaven, are those two points in which the earth's axis produced terminates in the heaven : 80