THE simplest form of the circulation of matter is that which is presented by the watery vapour contained in the atmosphere. From this vapour the dews and rains are formed which refresh the scorched plant and fertilize the earth. The depth of dew which falls we cannot estimate. On summer evenings it appears in hazy mists, and collects on leaf and twig in sparkling pearls; but at early dawn it vanishes again unmeasured-partly sucked in by plant and soil, and partly dispelled by the youngest sunbeams. But the yearly rain-fall is easily noted. In Britain it averages about thirty inches in depth; and in Western Europe generally, it is seldom less than twenty inches. Among the Cumberland mountains, in some places a fall of two hundred inches a year is not uncommon; while among the hills near Calcutta, as much as five hundred and fifty inches sometime falls within six months.
Now, as the whole of the watery vapour in the air, were it to fall at once in the form of rain, would not cover the entire surface of the Earth to a depth of more than five inches, how repeated must the rise and fall of this watery vapour be! To keep the air always duly moist, and yet to maintain the constant and necessary descent of dew and rain, the invisible rush of water upwards must be both great and constant. The ascent of water in this invisible form is often immediate and obvious, depending solely upon physical causes; but it is often①also in direct, and being the result of physiologicalchemical causes, is less generally perceptible. Thus: -or of1.Water circulates abundantly between earth and air through the agency of purely physical causes. We see this when a summer shower falling upon our paved streets is speedilylicked up again by the balmy winds, and wafted towards the region of clouds, ready for a new fall. But this form of circulation takes place on the greatest scale from the surface of the sea in equatorial regions, heated through the influence of the sun"s rays. Thence streams of vapour are continually mounting upwards with the currents of ascending air; and with these they travel north and south, till colder climates precipitate them in dew, rain, or snow. Returned to the arctic or the temperate seas by many running streams, these precipitated waters are carried back again to the equator by those great sea-rivers which mysteriously traverse all oceans; and. when there, are ready to rise again to repeat the same revolution. How often, since time began, may the waters which cover the whole Earth have thus traversed air and sea, taking part in the endless movements of inanimate nature!
