will be easily found: but if the earth be any other figure than that of a sphere, the length of a degree on the same meridian will be different in different latitudes, and if the figure of the earth resemble an oblate spheroid, the lengths of a degree will increase as the latitudes increase. The English translation of Maupertuis' figure of the earth, concludes with these words: (see page 163 of the work) "The degree of the meridian which cuts the polar circle being longer than a degree of the meridian in France, the earth is a spheroid flatted towards the poles." For, the longer a degree is, the greater must be the circle of which it is a part; and the greater a circle is, the less is its curvature. The first person who measured the length of a degree with any appearance of accuracy was Mr. Richard Norwood; by measuring the distance between London and York, he found the length of a degree to be 367196 English feet, or 691 English miles; hence, supposing the earth to be a sphere, its circumference will be 25020 miles, and its diameter 7964 miles ;* but if the length of a degree, at a medium, be 57069 toises, the circumference of the earth will be 24873 English miles, its diameter 7917 miles, and the length of a degree 69,1 miles. CONCLUSION. Notwithstanding all the admeasurements that have hitherto been made, it has never been demonstrated, in a satisfactory manner, that the earth is strictly a spheroid; indeed, from observations made in different parts of the earth, it appears that its figure : * 5280 feet make a mile, therefore 367196 divided by 5280 gives 69 miles nearly, which, multiplied by 360, produces 25020 miles, the circumference of the earth, but the circumference of a circle is to its diameter as 22 to 7, or more nearly as 355 to 113; hence, 355 113: 25020 miles: 7964 miles, the diameter of the earth. Again, 6 French feet hake one toise, therefore 57069 toises are equal to 342414 French feet; but 107 French feet are equal to 114 English feet hence, 107 F.f.: 114 E. f.:: 342414 F. f.: 364814 English feet, which, divided by 5280, the feet in a mile, gives 69.09 miles the length of a degree by the French admeasurement. Or, S42414, multiplied by 360, produces 123269040 French feet, the circumference of the earth, and 107: 114 :: 123269040: 131333369 English feet, equal to 24873.74 miles, the circumference of the earth, and 355 113:: 24873.74: 7917 miles, the diameter of the earth. : is by no means that of a regular spheroid, nor that of any other known regular mathematical figure; and the only certain conclusion that can be drawn from the works of the several gentlemen employed to measure the earth is, that the earth is something more flat at the poles than at the equator. The course of a ship, considering the earth a spheroid, is so near to what it would be on a sphere, that the mariner may safely trust to the rules of globular sailing, even though his course and distance were much more certain than it is possible for them to be. For which, and similar reasons, mathematicians content themselves with considering the earth as a sphere in all practical sciences, and hence the artificial globes are made perfectly spherical, as the best representation of the figure of the earth. CHAPTER IV. Of the Diurnal and Annual Motion of the Earth. THE motion of the earth was denied in the early ages of the world: yet, as soon as astronomical knowledge began to be more attended to, its motion received the assent of the learned, and of such as dared to think differently from the multitude, or were not apprehensive of ecclesiastical censure.- -The astronomers of the last and present age have produced such a variety of strong and forcible arguments in favour of the motion Robertson's Navigation, Book VIII. Art. 143. † Mr. Adam Walker, in his Familiar Philosophy (see the lecture on the momentum of light) supposes the earth and planets to be turned on their axes by the impulse of light! He says, "that in all the positions in which the earth stands to the sun, during its annual revolution round him, it will be found that more rays fall on one side of its axle and centre, than on the other." With all due respect to Mr. Walker's known abilities, I confess I do not see the truth of this assertion; if it be allowed, (as Mr. Walker has admitted in his 9th lecture on optics, page 403,) that the rays of light fall parallel upon the earth from the sun, will not these rays at the time of the equinoxes, strike the earth in a direction perpendicular to its axis? At this season, then, the impulse of light would have no effect whatever to turn the earth on its axis. of the earth, as must effectually gain the assent of every impartial inquirer. Among the many reasons for the motion of the earth, it will be sufficient to point out the following. 1. Of the Diurnal Motion of the Earth. The earth is a globe of 7964 miles in diameter (as has been shewn in Chap. III.) and by revolving on its axis 24 hours* from west to east, it causes an apevery parent diurnal motion of all the heavenly bodies from east to west. We need only look at the sun, or stars, to be convinced that either the earth, which is no more than a point when compared with the heavens, revolves on its axis in a certain time, or else the sun, stars, &c. revolve round the earth in nearly the same time. Let us suppose, for instance, that the sun revolves round the earth in 24 hours, and that the earth has no diurnal motion. Now, it is a known principle in the laws of motion, that if any 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, whatever curve it describes ; 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. But this never happens except on two days of the year, viz. at the time of the equinoxes, when the sun rises exactly in the east, and sets exactly in the west; for, in our summer, the sun rises to the north of the east, and sets to the north of the west; and in the winter it rises to the south of the east, and sets to the south of the west; and, therefore, its diurnal path divides the globe into two unequal parts; consequently, the sun does not move round the earth. To render this more intelligible to a young student, let a pin, of some inches in length, be fixed perpendicular upon an hori * That is, the time from the sun's being on the meridian of any place, to the time of its returning to the same meridian the next day; but the earth forms a complete revolution on its axis in 23 hours, 56 minutes, 4 seconds: see definition 61, page 13. † Dr. Keill, Lect 26. Emerson's Astronomy, page 11. zontal plane, and observe the shadow that the top of it describes on any day of the year; this shadow will always be a curve, except at the time of the equinoxes; hence, the earth is never in the sun's apparent diurnal orbit but then: for, if the top of the pin kept all the time in the plane of the sun's apparent diurnal orbit, the shadow described would be a straight line; because, it would fall in the intersection of two planes;t therefore, the sun has no diurnal motion round the earth, consequently, the earth has a diurnal motion on its axis. It is no argument against the earth's diurnal motion that we do not feel it; a person in the cabin of a ship, on smooth water, cannot perceive the ship's motion when it turns gently and uniformly round: neither does the motion of the earth cause bodies to fall from its surface; for all bodies, of whatever matter they are composed, are drawn to the earth by the power of its central attraction;§ which, laying hold of them according to their densities, or quantities of matter, without regard to their magnitudes, constitutes what we call weight. The phenomena of the apparent diurnal motion of the sun may be explained by the motion of the earth; thus, let IFGH (plate I. fig. v.) represent the earth, S the sun, and the circle DSBC the apparent concavity of the heavens. Let the earth revolve on its axis from I towards G (viz. from west to east.) Suppose a spectator to be at I, the sun, which is at an immense distance, and enlightens half the globe at once, will appear to be rising. As the earth moves round, the spectator is carried towards F, and the sun seems to increase in height: when he has arrived at F, the sun is at the highest. As the earth continues to turn round, the spectator is carried from F towards G, and the altitude of the sun keeps continually diminishing; when he has arrived at G, the sun is setting. During the time the spectator has been carried from I to G, the sun has * Emerson's Dialling, Prop. II. p. 9th. It is demonstrated in Euclid, Prop. III. Book XI. that if two planes intersect each other, their common section is a straight line. Ferguson's Astronomy, Art. 119. § Newton's Principia, Book III. Prop. vii. appeared to move the contrary way. Hence, it is evi, dent, that while the spectator is carried through the il luminated half of the earth, it is day-light; at the middle point F, it is noon; also, while he is carried through the dark hemisphere, it is night; and at H it is mid night. Thus, the vicissitude of day and night evidently appears by the rotation of the earth about its axis: what has been said of the sun is equally applicable to the moon, or any star placed at S; therefore, all the celes tial bodies seem to rise and set by turns, according to their various situations. The spectator at I, F, G, H, will always have his feet towards the centre of the earth, and the sky above his head, whatever position the earth may have; agreeably to the laws of gravitation or attraction. Thus, an inhabitant at a will be the most powerfully attracted towards his antipodes b, because, there is the greatest mass of earth under his feet in that direction; for the same reason b will be the most attracted towards a, m towards n, and n towards m, &c. Hence, it appears, that every body on the surface of the earth is attracted towards its centre, or rather towards the antipodes of that body, for the whole earth is the attracting mass, and not some unknown substance placed in the centre of the earth. There is no such thing as an upper and under side of the earth. Suppose a to be an inhabitant of Nankin in China, b will be an inhabitant of South America, near Buenos Ayres, each having the earth under his feet, and the sky above his head also, if n be an inhabitant a little east of Quito in South America, on the equator, m will be an inhabitant upon the equator in the island of Sumatra, and in the course of 12 hours, n will have the very same position as m by the revolution of the earth. 2. Of the Annual Motion of the Earth. The diurnal revolution of the earth on its axis being proved, the annual motion round the sun will be readily admitted; for, either the earth moves round the sun in a year, or else the sun moves round the earth: now, by the laws of centripetal force, if two bodies revolve about each other, they revolve round their common centre of |