THE EARTH. A difficult Subject of Investigation.-Form of the Earth.-How proved Globular.—Its Magnitude.Its annual Motion.-Elliptic Form of its Orbit.-Proofs of its annual Motion from the Theory of Gravitation.-From the Motion of Light.-The Earth's diurnal Motion.-Inequalities of Day and Night.-Weight of the Earth.-Maskelyne's Experiment.-Cavendish's Experiment.-Their Accordance.-Density of the Earth.-The Seasons-Calorific Effect of the Sun's Rays.-Why the longest is not the hottest Day-Why the shortest Day is not the coldest.-The hottest Season takes place when the Sun is farthest from the Earth.-Proofs of the diurnal Rotation.-Spheroidal Form of the Earth proved by Theory and by Observation. THE EARTH. LOCKE Somewhere observes, with his usual felicity of illustration, that the "nind, like the eye, while it makes us see and perceive all other things, can never turn its view with advantage upon itself." We encounter something similar to this in our researches through the universe; for of all the objects which compose it, one of the most difficult with which to obtain a complete and accurate knowledge is the planet which we inhabit. The cause of this is our proximity to it, and intimate connexion with it. We are confined upon its surface, from which we cannot separate ourselves. We cannot obtain a bird's-eye view of it, nor at any one time behold more than an insignificant portion of its surface. We have the same difficulty in obtaining an acquaintance with it that a microscopic animalcule would have in acquiring a perfect knowledge of the form and dimensions of a terrestrial globe twelve inches in diameter, on the surface of which it creeps. Still, by a variety of indirect methods supplied by the ingenuity of scientific research, we have been enabled to ascertain its form, and dimensions, and physical constitution, with a considerable degree of accuracy. FORM OF the earth. The first impression produced upon the eye of an observer, who has not carried his inquiries further, is, that the surface of the earth is a flat plane, interrupted only by the inequalities of the land. A little careful observation, however, upon the many phenomena which are easily accessible to every observer, will correct this erroneous impression. 1. It is well known that if a voyage were made upon the earth, continually preserving one and the same direction, or doing so as nearly as circumstances will permit, we should at length arrive at the place from which we departed. If the earth were an indefinite plane, this could not happen. It is evident, then, that whatever be the exact form of the earth, it is a body which is on every side limited, and one which must therefore have such a surface that a traveller or navigator can completely surround it in one continuous course. Let us see, however, whether we may not obtain evidence more distinct as to its form. If we stand on the deck of a ship at sea, and out of sight of land, the view being bounded only by sea and sky, and look at the horizon when a ship approaches, we shall at first see its topmast rising out of the water like a pole. As it gradually comes nearer to us, more of the mast will become visible, and the sails will be seen-cut off, however, horizontally, by the line at which the water and sky unite. Upon the nearer approach of the ship, the hull will at length become visible. Now, since this takes place on all sides around us, it will follow that when the ship is at a distance, there must be something interposed between the eye and it which intercepts the view of it; but as the surface of the water is generally uniform, and not subject to sudden and occasional inequalities like that of the land, we can only imagine its general form to be convex, and that its convexity is interposed between the eye and the object so as to intercept the view. Since the same effects are observed from whatever direction the ship may approach, it will follow that the same convexity must prevail on every side. If we admit the earth to be globular, or nearly so, and the surface of the water to partake of this figure, 1, the manner in which a ship becomes visible on approaching the eye will be easily and simply explained. In the position a, in the annexed figure, the convexity of the globe being between the ship and the eye, the view of it is intercepted; but as the ship approaches toward b, the masts first and then the sails and rigging rise above the line of sight and come into view, and lastly the hull will be seen. e; If, on the contrary, the surface extending from the eye to the ship were a plane, the ship would be rendered invisible only by reason of its distance whereas it is ascertained that a ship frequently is invisible at a distance at which it must be seen but for the interposition of some other object; this may be tested, and in fact is frequently tested at sea by mounting to the masthead, whence the seaman being enabled to overlook the convexity, sees vessels which are invisible from the deck, athough, strictly speaking, he is nearer to those vessels on the deck than at the masthead. When the mariner, after completing a long voyage, discovers by his observations and reckonings that he is approaching the desired coast, he ascends to the topmast and looks out for the appearance of mountains or other elevated land, and he invariably sees them from that point long before they are visible from the deck. He afterward sees them from the deck long before the general level of the country will be observed by him. All these are natural and necessary consequences of the convexity of the surface of the ocean. The same effects would be seen in any part of a continent which is sufficiently free from mountains and other inequalities. But we have a still more conclusive and convincing proof of the general form of the earth even than those which have been explained. When the moon passes directly behind the earth, so that the shadow which the earth projects behind it in the direction opposite to the sun shall fall upon the moon, we invariably find that shadow to be, not as is commonly said, circular, but such exactly as one globe would project upon the surface of another globe. Now, as this takes place always, in whatever position the earth may be, and while the earth is revolving rapidly with its diurnal motion upon its axis, it follows that the earth must either be an exact globe or so little different from a globe that its deviation from that figure is undiscoverable in its shadow. We may, then, consider it demonstrated that the earth may be practically regarded as globular in its form. We shall hereafter see that it slightly departs from the spherical figure, but our present purpose will be best answered by regarding it as a globe. The objection will doubtless occur to many minds that the inequality which exists on the surface of that portion of the globe that is covered by land, especially the loftier ridges of mountains, such as the Andes, the Alps, the Himalaya, and others, are incompatible with the idea of a globular figure. If the term globular figure were used in the strictest geometrical sense, this objection doubtless would have great force. But let us see the real extent of this presumed deviation from the globular form. The highest mountain on the surface of the globe does not exceed five miles above the general level of the sea. The entire diameter of the globe, as we shall presently see, is eight thousand miles. The proportion, then, which the highest summit of the loftiest mountains bears to the entire diameter of the globe will be that of five to eight thousand, or one to sixteen hundred. If we take an ordinary terrestrial globe of sixteen inches in diameter, each inch upon the globe will correspond to five hundred miles upon the earth, and the sixteen hundredth part of its diameter, or the hundredth part of an inch, will correspond to five miles. If, then, we take a narrow strip of paper, so thin that it would take one hundred leaves to make an inch in thickness, and paste such a strip on the surface of the globe, the thickness of the strip would represent upon the sixteen-inch globe the height of the loftiest mountain on the earth. We are then to consider that the highest mountainranges on the earth deprive it of its globular figure only in the same degree and to the same extent as a sixteen-inch globe would be deprived of its globular figure by a strip of paper pasted upon it the hundredth part of an inch thick. It is supposed that the greatest depth of the ocean which covers any portion of the globe does not exceed the greatest height of the mountains upon the land. If this be true, the ocean upon the earth might be represented by a film of liquid laid with a camel's-hair pencil upon the surface of a sixteen-inch globe. It is apparent, therefore, that depths and heights which appear to the common observer to be stupendous, are nothing when considered with reference to the magnitude of the earth; and that, so far as they are concerned, we may practically regard the earth as a true globe. THE MAGNITUDE OF THE EARTH. Having ascertained satisfactorily the figure of the earth, our next inquiry must be as to its magnitude; and since it is a globe, all that we are required to know is the length of its diameter. If a line were described surrounding the globe, so as to form a circle upon it, the centre of which should be at the centre of the globe, such a circle is called a great circle of the earth. Now if we know the length of the circumference of such a circle, we could easily calculate the length of its diameter, |