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it, without attempting to take measurements, till it became impossible to do so, owing to the clouds returning. Both observers received the sun's image through a telescope in a dark room upon a piece of white paper.

The transits of Venus are of special interest from their rarity and great physical importance. They occur at intervals of eight years, a small section of individual life; but alternating with intervals of more than a century, during which whole generations pass away, thrones crumble, and dynasties change. They are of importance as the very best means of ascertaining the distance and volume of the sun, which supply data for determining the distances and magnitudes of the planets, and serve as a universal standard of astronomical measurement. The second recorded transit, in 1761, was well seen; but the discordance of the results obtained at different stations shook faith in the accuracy of the observations. For the third and last, in 1769, many of the European governments sent costly expeditions to various parts of the globe; and among others, Captain Cook was despatched to witness the phenomenon at Tahiti. The next transit will occur December 9, 1874; and though not visible in this country, it will be watched elsewhere with an intensity of scientific solicitude which no natural incident has ever yet excited, and with much better instruments than have before been used. Another will follow after an interval of eight years, December 6, 1882, which will be seen in England, but only partially, commencing near sunset. Venus will not again be seen upon the sun's disk through the whole of the next century, or till June the 8th, 2004.

But little is known of the physical constitution of Venus, owing to the intense splendour with which she shines. The existence of a considerable atmosphere is inferred from the appearance of a penumbral light round the planet during her transits, as well as from a faint radiance observed to stretch beyond her directly illuminated hemisphere. The line in the annexed uppermost figure marks the boundary of the direct influence of the sun's rays; and the upper and lower projections beyond it show the twilight, which is referred to atmospheric reflection. Variable and fleeting spots have also been repeatedly noticed, as in the second figure, which naturally leads to the supposition of an atmosphere charged with clouds and vapours, with water upon the surface, from which they are formed. The conclusion that she has mountains and valleys rests upon the fact that the edge of her enlightened part appears shaded, that her corners are sometimes obtuse, and present a luminous point apparently detached from the planet. Schroeter regarded this as the summit of a high mountain, illuminated by the sun after he had ceased to be visible to the rest of that hemisphere. If these conclusions may be depended upon, and they are warranted by strong evidence, Venus presents striking points of analogy to the constitution of the earth. An atmosphere reflecting light, the medium of sound, and a highway for "fire and hail, snow and vapour," a superficies exhibiting the diversities of land and water, hill and vale- these are some of her probable attributes, features expressing a family likeness to our globe, and indicating the action upon her surface of that mighty upheaving agency, which, in bygone ages, piled the Alps, and reared the ramparts of the Himalaya.

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THE EARTH. We now come to the abode of Man the cradle, the home, and the grave of our race for a period of six thousand years-the third planetary body in point of distance from the central sun, and the first in the system of which we have certain knowledge of its being dignified with the presence of an attendant orb. The introduction of our globe to a place among celestial objects involves an apparent contradiction; yet such is its real character in the constitution of the universe, and such is its obvious aspect as viewed away from its surface. To rustic ignorance it will seem a statement palpably absurd, that any affinity exists between the earth and the stars in the firmament. They are mere points of light in the sky, and have no perceptible dimensions, whereas our world appears of immeasurable extent, and exhibits no luminosity like theirs to the eye of sense. It seems, too, a perfectly inert mass. There is no movement discernible independent of that of the rivers flowing in their channels, the seas tossing in their bed, and the forests bending to the gale, while the celestial bodies appear in constant procession from place to place in the concave of the heavens. There is nothing, however, more susceptible of demonstration than that the obvious state of our globe is not its actual condition-that the apparently quiescent habitation of mankind is an unceasing traveller in space-that its opaque mass exhibits the same luminous aspect to the nearer planets which they present to us-and that in structure and economy the earth is in fraternal relationship to the celestial host, and may be denominated, with perfect propriety, a star. Physical science, in the three departments of astronomy, geography, and geology, deals with the mass of our globe. The former is chiefly concerned with its figure and magnitude, its atmosphere and motions.

To the eye the earth appears an immense plain stretching out in all directions to an indefinite extent. This was the current opinion of mankind respecting its form in early times. But a few simple facts prove the suggestion of the senses here to be erroneous. The limit of vision to the traveller upon an extensive level, or to the mariner at sea, is a well-defined circle of which the observer is the centre; and it may be geometrically


proved, that this circular horizon is a certain indication of the circular figure of the body to which it relates. In any direction in which a ship leaves shore, or approaches the coast, the vessel is observed as if gradually sinking in the ocean, or rising from it

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:

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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.

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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

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