64 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



poiu' assists in at least five, perhaps six, ways to put air in motion 

 and produce winds. (1.) By evaporation the air is cooled ; by 

 cooling its specific gravity is changed, and, consequently, here is 

 one cause of movement in the au', 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 vapom^, 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 mnd is called for 

 below. (4.) Arrived in the cloud-region, this vapom^, being con- 

 densed, 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 smTOunding au', 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.) Innmnerable 

 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 bloAV 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 atmo- 

 sphere, 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. These agents, singly and together, produce rarefaction, 

 A channel of rarefied diminish pressuro, and call for an inward rush of air 

 and over SSf ""^ ^^^ either sidc. Mr. Espy asserts, and quotes actual 

 Stream. obscrvation 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 com^se. That those w^hich have their origin at sea, 

 on the other side of the Gulf Stream, do (§ 174) often make right 

 for it, is a fact well kno^vn to seamen. The Gulf Stream from 



