144 



FIRST YEAR SCIENCE 



of the air, except the high and low pressures, the transfer 

 would be in a straight line from one place to the other, 

 and it could always be told in what direction the high and 

 low pressures were, by direction of the wind. But ob- 

 stacles like mountains and hills deflect the air currents. 

 There are also other causes which influence the direction 

 of the movement; chief among these is the rotation of the 

 earth on its axis. 



71. Velocity and Effect of Wind Action. The velocity of 

 air movement varies from a gentle breeze which has not 



force enough to stir the 

 leaves, to the terrific and 

 almost irresistible blast 

 of the tornado, which 

 sometimes attains a ve- 

 locity of a hundred miles 

 an hour and sweeps 

 everything before it. 

 The velocity of ordinary 

 wind is measured by an 

 instrument, called an 

 anemometer, which usu- 

 ally consists of four 

 aluminum cups attached 

 by horizontal arms to a vertical spindle, the number of 

 revolutions of which is recorded on a dial by a train of 

 cogwheels geared to the spindle (Fig. 71). 



When the wind has great velocity, it can be estimated 

 only by the pressure which it exerts. A measure for 

 the velocity of wind which needs no apparatus is given 

 by Professor Hazen and is as follows : 



0. Calm. 



1. Light ; just moving the leaves of trees. 



2. Moderate ; moving branches. 



Fig. 71, 



