176 Winds and fhcir Causes* 



diately the sea-breeze sets in ; and during the night, the land 

 becoming cooler than the sea, from radiation of its solar heat 

 and evaporation, a contrary current, called the " land wind," 

 takes place. An easy experiment satisfactorily explains this 

 phenomenon : — " Take a large flat vessel, fill it with cold 

 water, and into the middle of this put a water-plate filled witli 

 warm water : the first will represent the ocean, the latter an 

 island rarefying the air above it. Blow out a wax candle ; and, 

 if the place be still, on applying it successively to every side 

 of the vessel, the fuliginous particles of the smoke, being visi- 

 ble and very light, will be seen to move towards the plate, 

 and, rising over it, point out the course of air from sea to land. 

 If the ambient water be warmed, and the plate filled with cold 

 water, when the wick is held over the centre of the plate, the 

 contrary will happen, and show the course of the wind from 

 land to sea." [Clare^ on Fluids.) 



The general breeze, or " trade wind," has been known to 

 bring ships sailing from the equator as far north as the Eng- 

 lish channel. But this seldom happens ; because the vast con- 

 tinent of Europe being generally warmer than the Atlantic 

 ocean, the prevailing winds set the contrary way. Hence 

 our equhioctial gales in the month of March, as well as winds 

 from that quarter the greater part of the year. The gales at 

 the vernal equinox may be accounted for thus : — 



The power of the sun is daily increasing; which, in con- 

 junction with the agricultural face of Europe, ploughed, har- 

 rowed, and rolled down to a smooth, naked, and reflecting 

 surface, so rarefies the air, that the cooler, and, consequently, 

 denser air over the western ocean rushes fiiriously eastward 

 in violent gales. These continue usually four or five days, or 

 till the aerial ocean attains an equilibrium. After this, the 

 surface of the earth is soon clothed in verdure, and no longer 

 so reflective ; the currents of air become variable, according 

 to circumstances near or remote, till the fields are shorn of 

 their vegetable covering, and the exposed summer-dried sur- 

 face again attracts the cooling gales from the westward at the 

 autumnal equinox. 



Different currents of air, in both hemispheres, are caused 

 by accidental and local rarefactions thereof, either from heat 

 or sudden and heavy falls of rain, and sometimes from vol- 

 canoes. These phenomena, wherever they happen, produce a 

 sort of vacuum, of less density than the surrounding atmo- 

 sphere ; which, closing to fill up the unoccupied or rarer space, 

 generates those streams of air, which, according to their force, 

 are called breezes or gales ; or, from their whirling direction, 

 hurricanes or tornadoes. 



