Page:Physical Geography of the Sea and its Meteorology.djvu/93

 Rh the air, as is manifest in the tendency of the cooled air to flow-off, and of warmer and lighter to take its place. (2.) Excepting hydrogen and ammonia, there is no gas so light as aqueous vapour, its weight being to common air in the proportion of nearly 5 to 8; consequently, as soon as it is formed it commences to rise; and, as each vesicle of vapour may be likened, in the movements which it produces in the air, to a balloon as it rises, it will be readily perceived how these vaporous particles, as they ascend, become entangled with those of the air, and so, carrying them along, upward currents are produced: thus the wind is called on to rush in below, that the supply for the upward movement may be kept up. (3.) The vapour, being lighter than air, presses it out, and, as it were, takes its place, causing the barometer to fall: thus again an in-rush of wind is called for below. (4.) Arrived in the cloud-region, this vapour, being condensed, liberates the latent heat which it borrowed from the air and water below; which heat, being now set free and made sensible, raises the temperature of the surrounding air, causing it to expand and ascend still higher; and so winds are again called for. Ever ready, they come; thus we have a fourth way. (5.) Innumerable rain-drops now begin to fall, and in their descent, as in a heavy shower, they displace and press the air out below with great force. To this cause Espy ascribes the gusts of wind which are often found to blow outward from the centre, as it were, of sudden and violent thunder-showers. (6.) Probably, and especially in thunder-storms, electricity may assist in creating movements in the atmosphere, and so make claim to be regarded as a wind-producing agent. But the winds are supposed to depend mainly on the power of agents (2), (3), and (4) for their violence.

178. A channel of rarefied air in the atmosphere and over the Gulf Stream.—These agents, singly and together, produce rarefaction, diminish pressure, and call for an inward rush of air from either side. Mr. Espy asserts, and quotes actual observation to sustain the assertion, that the storms of the United States, even those which arise in the Mississippi Valley, travel east, and often march out to sea, where they join the Gulf Stream in its course. That those which have their origin at sea, on the other side of the Gulf Stream, do (§ 174) often make right for it, is a fact well known to seamen. The Gulf Stream from Bimini to the Grand Banks is constantly sending up volumes of steam; this, being lighter than air, produces a channel way of rarefied air through