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 ocean. This current passes on, gradually widening and becoming less marked, till it is lost on the western shores of Europe. A less accurately defined under-current, from the poles, is constantly setting in towards the Equator, to supply the place of the heated water which takes the course already described. Besides rendering important aid to the navigator, these oceanic currents assist in mantaining [sic] an equilibrium of temperature on the earth, moderating the severity of the polar frosts, and tempering the sultry heats of the tropics.

Among the circumstances which materially affect the vaporization of liquids, one of the most important is atmospheric pressure. We have said that water boils at 212&deg;, but this statement requires some qualification, as the boiling point of water will vary according to the pressure of the atmosphere as indicated by the barometer. The aërial ocean which envelopes this planet presses upon the surface of the liquid ocean with a force equal to nearly fifteen pounds on every square inch; in other words, a column of air an inch square, extending from the level of the sea to the top of the atmosphere, weighs between fourteen and fifteen pounds. The elastic force of air is necessarily equal to its pressure. Let us try to make this point intelligible to the reader. If the mercury of a barometer stands at a height of about thirty inches in the open air, indicating a pressure of fifteen pounds, it will stand at exactly