THE WINDS 23 



we read the dials at the commencement and end of an hour 

 or day, we can obtain the number of miles of wind which 

 have passed the instrument in the hour or day. The former 

 is alluded to as velocit}' so many miles per hour ; the latter 

 as the total miles of wind in the day. These anemometers 

 should be small and light ; for otherwise short and strong 

 gusts of wind produce a momentum so great that the cups 

 and rods do not cease to rotate when the gusts stop, and 

 consequently the readings are too large. 



But besides the velocity, the force of the wind may be 

 measured by its pressure in pounds upon a square foot of 

 surface kept continually opposed to the wind by means of a 

 vane. There is, of course, an intimate connection between 

 the two the pressure is equal to the square of the velocity 

 divided by 300. In Table XIV. it will be seen that while the 

 pressures are very small for small velocities, they increase 

 rapidly as the velocities increase, until velocities of 100, 110, 

 and 120 miles per hour produce pressures which sweep away 

 trees and buildings. 



There is considerable doubt as to the accuracy of these 

 pressure-plate anemometers when the wind is violent ; the 

 clock-work recording apparatus is somewhat cumbersome, 

 and the recording pencil seems to be jerked forward so as to 

 indicate too high pressures. At the Kempshot observatory, 

 near Montego Bay, all the usual recording apparatus has been 

 removed from the pressure-plate anemometer, and a very light 

 needle moves an index forward, which remains at the highest 

 reading ; the instrument therefore registers the strongest 

 gusts onl}', and it registers them correctly as far as can be 

 ascertained. 



Another anemometer was devised a few years ago by Mr. 

 Dines, on the principle that when the wind blows over the 

 mouth of a pipe the pressure of the air within the pipe 

 is diminished. It works ver}^ well ; and it would be interesting 

 to compare the indications of the three instruments when the 

 wind is over 60 miles an hour. 



It must, however, often occur that instruments are not at 

 hand to measure the force or the velocity of the wind, and 

 consequently arbitrary scales are used. In Great Britain the 

 scale adopted was compiled especially for nautical purposes 



