have thus briefly enumerated the sources from which they are drawn. As the gravity of any body is only the aggregate attraction of all its particles, it is easy to conceive that there is a point within each body into which the whole attractive force of that body may be supposed to be concentrated, without producing any variation in the general and aggregate effect of that attraction; and this centre of attraction, under all general circumstances, is coincident with the centre of the mass, or centre of gravity of the body. By thus conceiving the attractions of two bodies to be reduced to those of two physical points, not only the investigations of all the problems into which such considerations enter, but all the conclusions drawn from such inves tigations, are greatly simplified; as by this means the precise places of these bodies are ascertained; the exact distance between them determined; and the indefinite number of forces, acting in different directions, are all reduced to two, acting in the same right line, and directly opposed to each other. As the Earth contains an incomparably greater quantity of matter than any of the other bodies immediately in its vicinity, the attractive forces of all these are wholly destroyed by that of the terraqueous globe, and each body descends towards its centre by the difference of its attraction and that of the Earth. This difference is denominated terrestrial gravitation, or, more generally and simply, gravity. It is not, however, intended by this term either to indicate the nature of gravity, or to assign its cause, but merely to mark the direction of its tendency. This direction is always towards a point which is near the centre of the Earth; or, more strictly speaking, that direction is always in lines perpendicular to the surface of the Earth, at the points where they would penetrate it. Bodies do not therefore descend in lines that are strictly parallel to each other; but as the spaces through which we can observe their descents are so extremely small compared with the distances of these bodies from the centre of attraction, these lines, or the tendencies of the bodies when at rest, and at small horizontal distances from each other, are always considered as parallel in physical researches; a consideration which simplifies the inquiry without sensibly affecting the accuracy of the result. Thus the action of two weights suspended from the extremities of a lever, and keeping it in equilibrio, are regarded as acting at right angles to the axis of that lever; and consequently the directions of their tendencies as being parallel to each other. A similar observation may also be made relative to the intensity of this force; for though this intensity varies inversely as the square of the distance from the attracting body, the spaces through which the descents of bodies can be observed are so small with respect to the terrestrial radius, that, considering the power of gravity as constant for those small spaces, can seldom, in any practical case, lead to an error in the result of an hundred thousandth part of the whole. For the Earth's radius being about 4000 miles, the force of gravity at its surface will be to that force at of a mile, or 105 feet aboye it, as 4000 (4000%)2 : (4000)2 :: 1: (4000.02 and hence (200000) * = .99999; 1-99999 = '00001= 2 the difference between the force of gravity at the Earth's surface and at this elevation above it. Now as the spaces are as the forces, and a heavy body near the surface of the Earth falls through a space equal to 16 feet, or 193 inches in the first se cond of its descent, it would fall through 192.99807 inches at the height of 105 feet above the surface. We shall therefore have 193-192.99807·00193 of an inch for the difference of the spaces fallen through in the first second at these heights; and consequently this difference, expressed in terms of the whole space, will be 00001. The force of gravity may, therefore, be regarded as constant, at least the difference for any heights at which we can make experiments is altogether insensible in the results, as the error would not exceed of an inch in a space of 830 feet. It is fully proved by experience, that all bodies, when perfectly at liberty to obey the action of gravity, fall equally swift, independent of the resistance of the medium through which they move; as a feather and a piece of gold in the exhausted receiver of an air-pump descend through the same space in the same time; and as this descent is not influenced by the magnitude, shape, or texture of the body, it proves that the effect of gravity is always proportional to the mass or quantity of matter in the bodies upon which it acts. Experience also proves that when bodies freely descend by the influence of this agent, they receive equal accessions of velocity in equal times; from which it follows that the action of gravity is not merely constant, as it regards duration, but that its intensity is also constant, whether the bodies upon which it acts are at rest or in motion. Both these properties, therefore, distinguish it from mechanical causes, the action of which is always proportional either to the magnitude or surface of bodies, and produces, in the same time, a less acceleration in the velocities when the bodies are in motion in the direction of their action, than when they are at rest at the moment this action com mences. The effects produced by the action of terrestrial gravity, and which are the subjects of constant obserS vation, may be arranged under two heads; first, as the bodies upon which its influence are to be considered are preserved at rest on the Earth's surface, or supported by any other means; or, secondly, as its effects are productive of either motion or change of motion. It is from effects of the first kind that bodies derive that tendency to descend, which we denominate weight; and without which the diurnal revolution of the Earth would cause them to be projected from its surface with velocities proportional to their respective situations. The velocity of those at the equator would exceed fifteen hundred feet per second; and, for other situations, the velocities would be proportional to the squares of the cosines of the latitude. The same power preserves the ocean within its proper boundaries; for by its action in directions every where perpendicular to the surface of the waters, they are caused to assume a globular form, and that general equilibrium between all parts of their mass is established, by which their characteristic sphericity is preserved, amidst all the varying causes by which they are perpetually agitated: and gravity thus becomes the perpetual fulfiller of that command which said to the mighty ocean, Hitherto shalt thou go, but no further.' It is the direction of this force, also, from whence we derive our ideas of upwards and downwards; the former of which is opposed to the tendency of its action, and the latter coincides with it; and, consequently, our notions relative to the inhabitants of distant regions of the globe being differently situated in this respect from ourselves, are mere prejudices, originating in our own limited views of the subject. [To be continued.] The Naturalist's Diary For AUGUST 1819, • While the Earth remaineth, seed time and harvest shall not fail.' Away desponding thoughts each doubt avaunt. Still, long as nature's Gop remains and rules, He fed them with rich Manna from the Skies! THE powerful influence of the solar rays now contributes to ripen the various sorts of grain, which are benevolently given for the food of man and cattle. Fine weather is very desirable, that the principal source of the farmer's wealth may be safely housed; for sudden storms beat down the nearly ripened corn, and materially injure it. The utmost diligence is now exerted, and labourers from all parts are eagerly engaged to give their assistance in this delightful occupation; all is bustle and activity. Poured from the villages, a numerous train Or bind in sheaves, or load, or guide the wain, Old age and infancy with careful hand The various kinds of wheat are described in our last volume, p. 209. To these we may add the Talavera, which may be sown with equal success in spring as in autumn, |