284 WELLS'S NATURAL PHILOSOPHY. 



countries, across the equator, and produces a south-west -wind. "When the 

 sun, on the other hand, has left the northern side of the equator for the 

 southern, the southern hemisphere is rendered hotter than the northern, and 

 the direction of the wind is reversed, or the monsoon blows north-east from 

 October to April. 



The monsoons are more powerful than the trade-winds, and very often 

 amount to violent gales. They are also more useful than the trade-winds, 

 since the mariner is able to avail himself of their periodic changes to go in 

 one direction during one half of the year, and return in the opposite directioa 

 during the other half 



What is th 627. In some parts of the world, as on coasts and islands, 



explanation of the heating action of the sun produces daily periodical winds, 

 breezes"*^ ^®* which are termed land and sea-breezes. 



During the day, the land becomes much more highly heated 

 by the sun than the adjacent water, and consequently the air resting upon 

 the land is much more heated and rarefied than that upon the water. The 

 cooler and denser air, therefore, flows from the water toward the land, con- 

 stituting a sea-breeze, and, displacing the warmer and lighter air over tho 

 land, forces it into a higher region, along which it flows in an upper current 

 seaward. 



At night a contrary effect is produced. After sunset the land cools much 

 more rapidly than tho water, and tho air over the shore becoming cooler, 

 and consequently heavier than that over the sea, flows toward the water and 

 forms the land-breeze.* 



The phenomena of land and sea-breezes may be well illustrated by a simple 

 experiment. FiU a large dish with cold water, and place in the middle of it 

 a saucer full of warm water ; let the dish represent the ocean, and the saucer 

 an island heated by the sun, and rarefying the air above it ; blow out a can- 

 dle, and if the air of the room be still, on applying it successively to every 

 side of the saucer, the smoke will be seen moving toward it and rising over 

 it, thus indicating the course of the air from sea to land. On reversing the 

 experiment, by filling the saucer with cold water, and the dish with warm, 

 the land-breeze will be shown by holding the smoking wick over tho edge 

 of the saucer ; the smoke wUl then bo wafted to the warmer air over the dish. 



, ^ , . 628. In the temperate zones, the winds have 



In what regions t i /. 



do variable httle of regularity, and these latitudes are 



winds prevail ? ^ "^ "^ ' 



known as the regions of "variable winds." 



In the tropics, the great aerial currents known as the trade-winds exist in 

 all their power, and control most of the local influences ; but in the temperate 

 zones, where the force of tho trade-winds is diminished, a perpetual contest 



• Advantage is taken of these breezes by coasters, which, drawing less water than 

 larirer vessels, can approach tho coast within those limits where the sea and land-breezes 

 first begin to operate. Thus a ship of war may not be able to take advantage of these 

 winds, while sloops and schooners may be moving along close to the shore under a press 

 of canvas, and be out of sight before the larger vessel is released from the calm bordering 

 these breezes, and fringing for some time the beach only. 



