282 WELLS'S NATURAL PHILOSOPHY. 



ascends, while a current of cold air rushes in to supply the vacancy. Two 



currents, the one of warm air flowing out, and the other of cold air flowing 



in, are thus continually produced; and to these movements of the atmosphere 



we apply the designation of wind. 



If the whole surface of the earth were covered with water, 



now do the tijQ -n-inds would always follow the sun, and blow uniformly 

 physical fea- j t j 



tnres of the from cast to west. The direction of the wind is, however, 



^nds*?^**^'*^^ continually subject to interruption from mountains, deserts, 



plains, oceans, etc. I 



Thus mountains which are covered with snow, condense and cool the air 

 brought in contact with them, and when the temperature of the current of 

 air constituting the wind is changed, its direction is liable to be changed also. 

 The ocean is never heated to the same degree as the land, and in conse- 

 quence of this, the general direction of the wind is from tracts of ocean to- 

 ward tracts of land. 



In those parts of the world which present an extended surface of water, 

 the wind blows with a great degree of regularity. 



What is the ^^"- Every variation exists in the speed of winds, from 



velocity _ and the niUdest zephyr to the most violent hurricane, 

 orce o win s ^ wind which is hardly perceptible moves with a velocity 



of about one mile per hour, and with a perpendicular force on one square foot 

 of '005 pounds avoirdupois. 



In a storm, the velocity of the wind is from 50 to 60 miles per hour, and 

 the pressure from T to 12 pounds per square foot. In some hurricanes, the 

 velocity has been estimated at from 80 to 100 miles per hour, with a varying 

 force of from 30 to 50 pounds. 



The force of the wind is ascertained by ob- 

 force of wind sGiving thc amouut of pressure that it exerts 



calculated ? . ■, p i • i , • , 



upon a given plane suriace perpendicular to its 

 own direction. 



If the pressure-plate acts freely upon spiral springs, the power of the wind 

 is denoted by the extent of their compression, wliich thus becomes a measure 

 of their force, the same as in weighing by the ordinary spring-balance. 



What is an -^° instrument for measuring the force of 



Anemometer? ^j^g wind is Called an Anemometer. 



How may winds 623. Wiuds mav be divided into threo 



be divided? classes : — Constant, Periodical, and Variable 

 winds. 



624. In many parts of the Atlantic and Pacific oceans, tho 

 trade-winds? wind blows with a uniform force and constancy, so that a ves- 

 sel may sail for weeks without altering the position of a sail 

 or spar. Such winds have received the designation of trade-winds, inasmuch 

 as they are most convenient for navigation, and always blow in one direction. 



