WIND. 7 



or velocity of the wind, both of which are quite independent 

 of the course along which, or rate at which, the storm 

 advances. 



It would, therefore, be erroneous to imagine that the velocity 

 of the wind during a gale, multiplied by the duration of the 

 gale, would give a correct idea of the extent of such disturbance ; 

 otherwise a gale of, say, three days' duration, with an average 

 velocity of 50 miles per hour, would have travelled a distance 

 of 3600 miles, whereas the actual distance traversed would 

 probably not exceed one-third of this, or it might even be very 

 much less. 



The direction and velocity of wind are most conveniently 

 recorded by means of graphic wind-gauges, or anemographs, in 

 which a slewing motion, for indicating direction, is combined 

 with the ordinary cup anemometer by means of tangent screws 

 actuated by fans. 



The lines indicating direction and velocity are drawn upon 

 a paper attached to a cylinder, which derives its motion from 

 clockwork. 



Some of these machines give continuous diagrams, so that 

 the direction and velocity of the wind at any moment during 

 the day or night may be read off at leisure. 



Another description of wind-gauge is that in which pressure, 

 instead of velocity, is recorded; but as yet these have not 

 proved at all reliable. 



RELATION OF WIND VELOCITY TO PKESSUKE. 



At first sight, it would appear an easy matter to establish a 

 comparison between the velocity of wind and the pressure 

 exerted by it; nevertheless, up to the present time no satis- 

 factory method of doing this has been devised. 



The chief difficulty seems to lie in the fact that the apparent 

 pressure per unit of area varies with almost every size and form 

 of surface against which the wind impinges. This is doubtless 

 in a great measure due to the varying degree of vacuum which 

 the wind current creates in the rear of the objects acted upon. 

 However this may be, all the best authorities on the subject 

 are agreed that, at present, there exists no satisfactory method 

 of establishing a relation between wind velocity and pressure ; 

 indeed, it seems doubtful if such a comparison be possible. 



