Although there are many places in the great Atlantic and Pacific Oceans where the depth of water is very great, yet it has been deduced from principles that are not liable to much error, that the general or average depth does not exceed three miles. It may appear very strange that we can assert any thing positive about the depth of water in those seas, that are to the lines used for sounding quite unfathomable; but it is effected very simply. Every person has seen a wave advancing along the level surface of a canal, and by observing with a watch, it could easily be found to move more quickly at some times than at others. The deeper any part of the canal is, the more rapidly does the wave move along; and partly by experiment, and partly by reasoning, the connection between the depth of the water and velocity of the wave has been discovered. Now, the tide which comes to Dublin every twelve hours is produced by the influence of the sun and moon on the vast body of water in the Southern Pacific Ocean; and the great wave there formed turns round Cape Horn, and passes up the Atlantic Ocean, to arrive at the coasts of Europe and North America. The time occupied by this great wave in passing from one end to the other of the Atlantic can thus be known, and, precisely as in a canal, the depth of water thus calculated. The circumference of the earth at its widest part is about 25,000, and its diameter 8000 miles. Hence the sheet of water which constitutes the ocean forms but 3-4000ths of its thickness, or nearly the same proportion as if we took an eighteen inch globe, and having spilled water on its surface, allowed all the excess of water to drain off. The remaining wetness would represent pretty nearly the condition of the waters of the ocean on the surface of the earth. By this means we can form, though obscurely, to our minds, an idea of the great magnitude of the earth itself. This magnitude renders also very inconsiderable those inequalities on the surface of the earth which constitute our highest ridges of mountains. A true model of Mont Blanc, the highest of European mountains, if constructed on the eighteen inch globe before referred to, would be unfelt by a finger drawn along its surface, and it would be only some of the highest peaks of the Andes and Himalayah that could be distinctly felt. Where man also employs his most gigantic energies and greatest efforts of skill to penetrate below the surface, forming mines by which the supplies of coal, of iron, of copper, and other minerals, have been obtained from the earliest times, the cavities that he makes can only be compared with the trace given by the point of a pin that had lightly touched the globe, and which would require a favourable incidence of light to see.