550 feet distant from, the other. The former was called the Piraic, and the latter the Phaleric wall; and the entire peribolus of fortifications connecting and surrounding the city and its harbours, must have been about 20 miles in

extent.

OF THE

FIGURE, MAGNITUDE, AND MOTIONS

OF THE

EARTH AND MOON,

AND OF THE

SOLAR SYSTEM,

OF WHICH THEY FORM A PART.

IT has been already asserted (p. 6) that the figure of the earth which we inhabit is nearly that of? A globe or sphere. How does this appear? The appearance of ships as they approach, or recede from, one another at sea; and, also, as they approach, or recede from, the shore, proves that the surface of the waters with which the earth is overlaid is, at least, convex; and the circumstance of the world's being circumnavigable-either by an easterly or a westerly route-proves that it is round, at least, in the direction of east and west; but the level necessary to be observed in making a long canal, which must be taken in the arc-not on the tangent of a circle; and, above all, the shadow of the earth-always circular-as seen on the surface of the moon in the time of a lunar eclipse, renders the doctrine of the sphericity of the terraqueous globe absolutely certain. How is the magnitude of the earth ascertained? To a person on the equator it is obvious that the polar star (the celestial pole) would appear on the (true) horizon; and it is also evident that, if he travelled northwards, that star would appear to ascend in the heavens in the same proportions; so that his latitude and its altitude would always be the same ;-now, if he were to travel due

north till he found that the polar star had increased one degree in altitude, it is evident that he would have moved one degree northwards on the earth's surface :—a degree, thus determined, has been frequently measured, and with unvarying results (in the same latitude-between 69 and 70 English miles) ;—and having thus found the length of a degree of a great circle of the globe, its circumference, diameter, superficies, and solid content, can be easily found. It has been found, however, from actual measurement, that degrees of the meridian are rather greater (i.e., parts of the circumference of a larger circle) towards the poles than near the equator; and from this it follows that? The earth must be somewhat flattened towards the poles (inasmuch as the greater the circle the less is its curvature), and protuberant at the equator.

The earth has two motions, namely? One round its own axis in a day, producing the vicissitudes of day and night, called its diurnal motion; and another round the sun in a year, producing the vicissitudes of the seasons, and called its annual motion.

How does it appear that the vicissitudes of day and night are brought about by the diurnal revolution of the earth on its axis? It is obvious, in order to produce this effect, either that the earth must perform a daily revolution on its axis, towards the east; or, that all the celestial bodies (compared with the magnitude of which the earth is little more than a point) must (in great or small parallel circles according to their distances from the celestial poles) revolve round the earth in the direction of the west, in the same time :-now, not only is this a supposition altogether at variance with that beautiful simplicity which is found to exist in all the works of the Creator; but, besides, it is a known principle in the "laws of motion" that if one body revolve round another as its centre, it is necessary that the central body be always in the plane in which the revolving body moves; therefore, if the sun move round the earth in a day, its diurnal path must always describe a circle which will divide the earth into two equal hemispheres, which it never does but at the time of the equi

noxes, when the sun rises exactly in the east, and sets exactly in the west; and consequently the sun does not

move round the earth. How happens it that we are not sensible of the motion resulting from the revolutions of the earth on its axis? Because it takes place in vacuo, is perfectly smooth and uniform, because the atmosphere moves along with it; and because, at every point of the earth's surface, all bodies are attracted towards its centre. By that universal attraction (of gravitation) by which? "Every particle of matter in the universe attracts every other particle, with a force directly proportioned to the mass of the attracting particle, and inversely to the square of the distance between them." And this tendency of bodies at the earth's surface to move towards its centre it is that constitutes ? Their gravity or weight. Does the central attraction of the earth arise from any attractive power lodged in the centre more than in any other part of the earth? No, but only because a greater mass of matter lies in that direction than in any other. How many degs. per hour is a person on the earth's surface carried along by the earth's diurnal rotation on its axis? The 24th part of 360, ie., 15 degs. And for every 15 degs. that any place on the earth's surface lies to the east of us, the inhabitants of that place have the sun? An hour before us in the morning. And for every 15 degs. that any place lies to the west of us, the inhabitants of that place have the sun? An hour later than we. St. Petersburgh, 30 degs. to the E. of us, has the sun? Two hours before us. Calcutta, 90 degs. to the E. of us? Six hours earlier. Barbadoes, 60 degs. to the W. of us? Four hours later. And Otaheite, 150 degs. to the W. of us? Ten hours later. How does it appear that the earth has also an annual motion round the sun? Because, if by means of a telescope, the sun's place in the heavens (in relation to a fixed star) be observed at any instant and carefully noted, the next day, at the same hour, he will appear to have moved a degree towards the east, and thus, in the course of a year, to have completed a great circle of the heavens; it is clear, therefore, either that the earth moves round the

sun in a year, or, that the sun moves round the earth in the same time. But, if two bodies revolve about each other, they revolve round their common centre of gravity; and, as the sun so greatly exceeds the earth in magnitude, that the common centre of gravity of both is constantly within the body (not very far indeed from the centre) of the sun; the earth, therefore? Revolves round the sun as its centre of motion. How does the earth's annual motion round the sun produce the vicissitudes of the seasons, together with the ever varying changes in the length of the day and night? If the earth's axis were at right angles to the plane of its orbit, the sun's apparent path in the heavens would always be in the plane of the equator, ie. in a great circle of the heavens encompassing the earth, everywhere equidistant from the celestial poles, and there would be no variation in the seasons, and no difference in the length of the day and night; but, as the earth's axis is inclined to the plane of its orbit at an angle of 66 degs., and is carried round the sun parallel to itself, always directed towards the same vanishing point (the celestial pole) in the sphere of the fixed stars—that is to say, preserves at all times the same direction as if the orbitual movement had no existence, it follows that the sun's apparent path declines to the extent of 23 degs. from each side of the plane of the equator; and, consequently, the northern hemisphere, to that extent, during one period, and the southern hemisphere during another period of the year, inclines to the sun; so that when it is summer in the northern, it is winter in the southern hemisphere, and conversely. How is the motion of the earth in its orbit maintained and regulated? By a nice combination and adjustment of the centripetal and centrifugal forces. What do you understand by the centripetal force (or solar gravitation)? The tendency which the earth has to move towards the centre of the sun. And by the earth's centrifugal force? The projectile (or tangential) force originally impressed upon it by the Creator. Under the joint operation of which two forces, the earth, in its annual path round the sun, realises the two first of those three remarkable "laws"