72 SCENERY OF TUB HEAVENS. 



Sir John Hcrschel 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 twenty- 

 five 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 move 

 ment 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 cur 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 move 

 ment 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- 



