If we include in the tropical seas all that part of the ocean where the surface temperature never falls below 68° F., and where consequently living coral reefs may occur, we find that it nearly equals in size the temperate and cold ocean-regions added together. This distribution of the waters over the surface of the globe is of the highest importance to mankind; for the immense extent of the tropical ocean, where, of course, the strongest evaporation takes place, furnishes our temperate zone with the necessary quantity of rain, and tends by its cooling influence to diminish the otherwise unbearable heat of the equatorial lands. The circumstance of ice being lighter than water also contributes to the habitability of our earth. Ice is a bad conductor of heat; consequently it shields the subjacent waters from the influence of frost, and prevents its penetrating to considerable depths. If ice had been heavier than water, the sea-bottom, in higher latitudes, would have been covered with solid crystal at the very beginning of the cold season; and during the whole length of the polar winter, the perpetually consolidating surface-waters would have been constantly precipitated, till finally the whole sea, far within the present temperate zone, would have formed one solid mass of ice. The sun would have been as powerless to melt this prodigious body, as it is to dissolve the glaciers of the Alps, and the cold radiating from its surface would have rendered all the neighbouring lands uninhabitable. * * * * * The mixture of the water of rivers with that of the sea presents some hydrostatic phenomena which it is curious enough to observe. Fresh water being lighter, ought to keep at the surface, while the salt water, from its weight, should form the deepest strata. This, in fact, is what Mr. Stephenson observed in 1818 in the harbour of Aberdeen at the mouth of the Dee, and also in the Thames near London and Woolwich. By taking up water from different depths with an instrument invented for the purpose, Mr. Stephenson found that at a certain distance from the mouth the water is fresh in the whole depth, even during the flow of the tide, but that a little nearer the sea fresh water is found on the surface, while the lower strata consist of sea water. According to his observations it is between London and Woolwich that the saltness of the bottom begins to be perceptible. Thus, below Woolwich the Thames, instead of flowing over a solid bed, in reality flows upon a liquid bottom formed by the water of the sea, with which no doubt it is more or less mixed. Mr. Stephenson is of opinion that, at the flow of the tide, the fresh water is raised as it were in a single mass by the salt water which flows in, and which ascends the bed of the river, while the fresh water continues to flow towards the sea.