of the western edge of her disc is visible, which is invisible at her setting, and the contrary takes place with regard to her eastern edge. There are also librations arising from the relative positions of the earth and moon in their respective orbits; but as they are only optical appearances, one hemisphere will be eternally concealed from the earth. For the same reason, the earth, which must be so splendid an object to one lunar hemisphere, will be forever veiled from the other. On account of these circumstances, the remoter hemisphere of the moon has its day a fortnight long, and a night of the same duration, not even enlightened by a moon, while the favored side is illuminated by the reflection of the earth during its long night. A planet exhibiting a surface thirteen times larger than that of the moon, with all the varieties of clouds, land, and water coming successively into view, must be a splendid object to a lunar traveler in a journey to his antipodes. The great height of the lunar mountains probably has a considerable influence on the phenomena of her motion, the more so as her compression is small, and her mass considerable. In the curve passing through the poles, and that diameter of the moon which always points to the earth, nature has furnished a permanent meridian, to which the different spots on her surface have been referred, and their positions are determined with as much accuracy as those of many of the most remarkable places on the surface of our globe.

The distance and minuteness of Jupiter's satellites render it extremely difficult to ascertain their rotation. It was, however, accomplished by Sir William Herschel from their relative brightness. He observed that they alternately exceeded each other in brilliancy, and, by comparing the maxima and minima of their illumination with their positions relatively to the sun and to their primary, he found that like the moon the time of their rotation is equal to the period of their revolution about Jupiter. Miraldi was led to the same conclusion with regard to the fourth satellite, from the motion of a spot on its surface.

SECTION X.

Rotation of the Earth invariable-Decrease in the Earth's Mean Temperature-Earth originally in a State of Fusion-Length of Day constantDecrease of Temperature ascribed by Sir John Herschel to the Variation in the Eccentricity of the Terrestrial Orbit-Difference in the Temperature of the Two Hemispheres, erroneously ascribed to the Excess in the Length of Spring and Summer in the Southern Hemisphere; attributed by Mr. Lyell to the Operation of existing Causes-Three Principal Axes of Rotation-Position of the Axis of Rotation on the Surface of the Earth invariable Ocean not sufficient to restore the Equilibrium of the Earth if deranged-Its Density and Mean Depth-Internal Structure of the Earth.

THE rotation of the earth, which determines the length of the day, may be regarded as one of the most important elements in the system of the world. It serves as a measure of time, and forms the standard of comparison for the revolutions of the celestial bodies, which by their proportional increase or decrease would soon disclose any changes it might sustain. Theory and observation concur in proving that among the innumerable vicissitudes which prevail throughout creation, the period of the earth's diurnal rotation is immutable. The water of rivers, falling from a higher to a lower level, carries with it the velocity due to its revolution with the earth at a greater distance from the center; it will therefore accelerate, although to an almost infinitesimal extent, the earth's daily rotation. The sum of all these increments of velocity arising from the descent of all the rivers on the earth's surface would in time become perceptible, did not nature by the process of evaporation raise the waters back to their sources; and thus, by again removing matter to a greater distance from the center, destroy the velocity generated by its previous approach; so that the descent of rivers does not affect the earth's rotation. Enormous masses projected by volcanos from the equator to the poles, and the contrary, would indeed affect it, but there is no evidence of such convulsions. The disturbing action of the moon and planets, which has so powerful an effect on the revolution of the earth, in no way influences its rotation. The constant friction of the trade-winds on the

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mountains and continents between the tropics does not impede its velocity, which theory even proves to be the same as if the sea together with the earth formed one solid mass. But although these circumstances be insufficient, a variation in the mean temperature would certainly occasion a corresponding change in the velocity of rotation. In the science of dynamics it is a principle in a system of bodies or of particles revolving about a fixed center, that the momentum or sum of the products of the mass of each into its angular velocity and distance from the center is a constant quantity, if the system be not deranged by a foreign cause. Now since the number of particles in the system is the same whatever its temperature may be, when their distances from the center are diminished their angular velocity must be increased, in order that the preceding quantity may still remain constant. It follows then that as the primitive momentum of rotation, with which the earth was projected into space must necessarily remain the same, the smallest decrease in heat by contracting the terrestrial spheroid would accelerate its rotation, and consequently diminish the length of the day. Notwithstanding the constant accession of heat from the sun's rays, geologists have been induced to believe from the fossil remains, that the mean temperature of the globe is decreasing.

The high temperature of mines, hot springs, and above all the internal fires which have produced and do still occasion such devastation on our planet, indicate an augmentation of heat toward its center. The increase of density corresponding to the depth and the form of the spheroid being what theory assigns to a fluid mass in rotation, concurs to induce the idea that the temperature of the earth was originally so high as to reduce all the substances of which it is composed to a state of fusion or of vapor, and that in the course of ages it has cooled down to its present state; that it is still becoming colder, and that it will continue to do so till the whole mass arrives at the temperature of the medium in which it is placed, or rather at a state of equilibrium between this temperature, the cooling power of its own radiation, and the heating effect of the sun's rays..

Previous to the formation of ice at the poles, the ancient lands of northern latitudes might no doubt have been capable of producing those tropical plants preserved in the coal-measures, if indeed such plants could flourish without the intense light of a tropical sun. But even if the decreasing temperature of the earth be sufficient to produce the observed effects, it must be extremely slow in its operation; for in consequence of the rotation of the earth being a measure of the periods of the celestial motions, it has been proved that if the length of the day had decreased by the three-thousandth part of a second since the observations of Hipparchus two thousand years ago, it would have diminished the secular equation of the moon by 4"-4. It is therefore beyond a doubt that the mean temperature of the earth cannot have sensibly varied during that time. If then the appearances exhibited by the strata are really owing to a decrease of internal temperature, it either shows the immense periods requisite to produce geological changes, to which two thousand years are as nothing, or that the mean temperature of the earth had arrived at a state of equilibrium before these observations.

However strong the indications of the primitive fluidity of the earth, as there is no direct proof of it, the hypothesis can only be regarded as very probable. But one of the most profound philosophers and elegant writers of modern times has found in the secular variation of the eccentricity of the terrestrial orbit an evident cause of decreasing temperature. That accomplished author, in pointing out the mutual dependencies of phenomena, says, "It is evident that the mean temperature of the whole surface of the globe, in so far as it is maintained by the action of the sun at a higher degree than it would have were the sun extinguished, must depend on the mean quantity of the sun's rays which it receives, or which comes to the same thing-on the total quantity received in a given invariable time; and the length of the year being unchangeable in all the fluctuations of the planetary system, it follows that the total amount of solar radiation will determine, cæteris paribus, the general climate of the earth. Now, it is

not difficult to show that this amount is inversely proportional to the minor axis of the ellipse described by the earth about the sun (N. 140), regarded as slowly variable; and that, therefore, the major axis remaining, as we know it to be constant, and the orbit being actually in a state of approach to a circle, and consequently the minor axis being on the increase, the mean annual amount of solar radiation received by the whole earth must be actually on the decrease. We have therefore an evident real cause to account for the phenomenon." The limits of the variation in the eccentricity of the earth's orbit are unknown. But if its ellipticity has ever been as great as that of the orbit of Mercury or Pallas, the mean temperature of the earth must have been sensibly higher than it is at present. Whether it was great enough to render our northern climates fit for the production of tropical plants, and for the residence of the elephant and other animals now inhabitants of the torrid zone, it is impossible to say.

Of the decrease in temperature of the northern hemisphere there is abundant evidence in the fossil plants discovered in very high latitudes, which could only have existed in a tropical climate, and which must have grown near the spot where they are found, from the delicacy of their structure and the perfect state of their preservation. This change of temperature has been erroneously ascribed to an excess in the duration of spring and summer in the northern hemisphere, in consequence of the eccentricity of the solar ellipse. The length of the seasons varies with the position of the perihelion (N. 64) of the earth's orbit for two reasons. On account of the eccentricity, small as it is, any line passing through the center of the sun divides the terrestrial ellipse into two unequal parts, and by the laws of elliptical motion the earth moves through these two portions with unequal velocities. The perihelion always lies in the smaller portion, and there the earth's motion is the most rapid. In the present position of the perihelion, spring and summer north of the equator exceed by about eight days the duration of the same seasons south of it. And 10,492 years ago the southern hemisphere enjoyed the advantage we now possess