But a little consideration will convince us that the circulation of the air and waters of the globe are themselves dependent upon the existence of those irregularities of the land-surfaces which they are constantly tending to destroy. Without elevated mountain ridges the regular condensation of moisture, and its collection and distribution in streams and rivers over every part of the land surfaces, could not take place. Under these circumstances the unchecked evaporation of the oceanic waters would probably go on, till the proportion of water-vapour increased to such an extent in the atmosphere as effectually to destroy those nicely-balanced conditions upon which the continued existence of both vegetable and animal life depend. But the repeated upward and downward movements which have been shown to be going on in the great land-masses of the globe, giving rise in turns to those lateral thrusts and tangential strains to which mountain-chains owe their formation, afford a perfect compensation to the action of the external forces ever operating upon the earth's surface. If, however, the uncompensated effect of the external forces acting on the earth's crust is calculated to bring about the destruction of those conditions upon which the existence of life depends, the uncompensated effect of the internal forces acting on the earth's crust are fraught with at least equal dangers to those necessary conditions. [Sidenote: CONTRAST BETWEEN THE EARTH AND MOON.] In our nearest neighbour among the planets--the moon--the telescope has revealed to us the existence of a globe, in which the internal forces have not been checked and controlled by the operation of any external agencies--for the moon appears to be destitute of both atmosphere and water. Under these circumstances we find its surface, as we might expect, to be composed of rocks which appear to be entirely of igneous origin; the mountain-masses, unworn by rain or frost, river or glacier, being of most prodigious dimensions as compared with those of our own globe, while no features at all resembling valleys, or plains, or alluvial flats are anywhere to be discerned upon the lunar surface. But by the admirable balancing of the external and internal forces on our own globe, the conditions necessary to animal and vegetable existence are almost constantly maintained, and those interruptions of such conditions, produced by hurricanes and floods, by volcanic outbursts and earthquakes, may safely be regarded as the insignificant accidents of what is, on the whole, a very perfectly working piece of machinery.