evidence of the convexity of the water between the eye and the object. We also find, that during a lunar eclipse the shadow projected by the earth upon the disk of the moon is always of a circular shape. The common occurrence now of a voyage round the world, proceeding in the same general direction, east or west, and arriving at the same point again, demonstrates the figure of the earth to be either that of a sphere or a cylinder; and the latter is disproved, and the convexity of the surface shown, north and south, by the gradual declination or rise of the north and south circumpolar stars, as the equator is approached or receded from. Our terrestrial mansion, therefore, is a vast mass of matter of a spherical form like the planets whose round disks are the objects of telescopic observation. The spherical figure of the planetary bodies-a result of the law of gravitation-is, on many accounts, the best shape they could have assumed. The same phenomena could not have been offered to their surfaces, with the same machinery, supposing any other form. Had the earth been a rotating cylinder, the solar beams could not have reached its two extremities together, or its general superficies with either extremity. But it is only an approximation to the truth of its actual shape, to speak of our world as having a spherical form. It is not a globe whose circumference is everywhere at an equal distance from the centre, a property essential to a sphere. A process of reasoning led Newton to the conclusion, that the circle of the earth's daily rotation upon its axis being the greatest at the equator, the consequent greater action there of the centrifugal force would produce a bulging out of the surface in the equatorial regions of a yielding mass, and a flattening at the poles; and this deduction from the laws of forces has been proved to be correct by the actual measurement of the lengths of degrees of the meridian, made with care and precision by the commissioners of various nations. The certain conclusion obtained is, that our globe is an oblate spheroid, an orange-shaped ball, compressed at the poles, and elevated at the equator, having the following dimensions:

The length of the axis of the poles is thus about twenty-six miles and a half less than the diameter of the equator. It is highly improbable that any error of importance exists in this measurement, founded upon the principle first employed by Eratosthenes, when he attempted to determine the value of an arc of the meridian between Alexandria and Syene.

Sir John Herschel considers it unlikely that an error to the extent of five miles can subsist in the diameters; and the equatorial diameter gives an extent of nearly twentyfive thousand miles, accurately 24,899, as the value of the equatorial circumference.

The earth has two principal motions, a rotation upon its own axis, and a translation in space. We can obviously have no ocular evidence of the diurnal rotation like that which we obtain in the case of the sun and some of the other planets, by observing the movement of spots upon their surface. But we have ample proof of the fact. There is absurdity upon the face of the ancient doctrine, that the daily apparent procession of the heavenly bodies round the earth is a real progress-that a point utterly insignificant when compared with the general aggregate of stars is a centre around which they circulate; and when we think of the inconceivable velocity with which they must travel, in order to compass the immeasurable circles which in that case they describe, the absurdity heightens. The rotation of our globe is not, however, a doctrine based on probabilities. The experiment of falling bodies descending in advance of the plumb line is direct and positive demonstration of the fact, corroborated by the diminution of the force of gravity at the equator. According to Laplace, the chances are eight thousand to one that the earth so revolves. The rate of the earth's rotation at the equator, where the circle of the circumference is the greatest, is about sixteen miles a minute. Its velocity, at thirty degrees of latitude, which is below the most southerly point of Europe, is computed at fourteen miles in the same time; and at forty-five degrees, or about the centre of France, it is eleven miles. Laplace has discussed the point with great care, whether the rate of the diurnal rotation is liable to be perturbed, and the time of revolution affected by the influence of volcanoes, earthquakes, winds, and currents in the ocean, and has demonstrated their effects to be altogether insensible. He has also examined the question, whether a variation of the mean temperature of the globe may not have influenced the velocity of rotation, and altered the length of the day. The temperature at the bottom of deep mines indicates a central heat. Geological appearances also intimate a large portion of the crust of the globe to have once been in a state of fusion, and it is a well-known property of heat to cause the expansion of the substances into which it enters. Allowing therefore a former very high temperature, the contraction of the terrestrial spheroid would be a consequence of its cooling down, the diminution of its volume without altering its mass, through the molecules approximating to the centre, causing thereby some change of velocity in the superficial rotation. We have no reason, however, to suppose, that any diminution of temperature has occurred, since man has existed upon the soil, sufficient to produce a sensible alteration in the length of his day and night. All history proclaims its uniform duration, age after age; and Laplace, who first started this speculation, came to the conclusion, that since the time of Hipparchus, the length of the day has not been affected by the two-hundredth part of a centesimal second. How beautiful the arrangement of the diurnal revolution of our terrene mansion! How benign the results! The alternation of light and darkness—the gorgeous sunrise the resplendent noon-the calm glory of the eventide the absorption and radiation of heat-and the trade winds, upon whose uniform direction and constant action the navigator reckons on the breast of the ocean.

The other principal motion of our globe is its translation in space. This appeals not to our senses like the orbital movements of the surrounding planets, but it is supported by irrefragable evidence. It accounts for the phenomenon of the apparent annual revolution of the sun, as an actual transit in a vessel on the water accounts for the apparent movement of the banks of a river, or the shore of the sea. It satisfactorily explains the seeming anomalies of the planetary paths. It has received direct confirmation from the aberration of the stars, and may be regarded as established on the firm basis of demonstra