84 TOWERS AND TANKS FOR WATER-WORKS. 



does not exist; therefore the exact wind movement cannot be 

 reduced. Occasionally, during the thunder-storms and gusts 

 of summer, anemometers will record for a brief period velocities 

 up to 75 miles an hour; but in such cases the storms and gusts 

 are of short duration. It is, however, reported that at some 

 seacoast stations and at Mount Washington, N. H., velocities 

 as great as 100 miles per hour have been continuously recorded 

 during pronounced storms. Occasionally during storms, sudden 

 and violent gusts of wind occur, considerably greater than the 

 indicated or mean velocities. The impact of such gusts upon 

 engineering structures are likely to set up coincident vibration 

 out of proportion to the effect of the recorded velocities of the 

 wind. In an article in the Engineering News, Dec. 13, 1890, 

 Prof. Marvin states that momentary pressures as great as 35% 

 in excess of the recorded mean pressures may continually occur 

 and recur, and if their rate of occurrence be at all synchronous 

 with the natural time of vibration of the structure or any part 

 thereof, remarkable results may follow. The greatest velocity 

 so far registered is reported from the signal station at Point 

 Reyes, Cal., where on May 18, 1902, a wind velocity of 102 

 miles an hour was registered, and for several moments the 

 anemometer recorded a velocity of 120 miles an hour, the 

 violence of the storm finally ripping the cups from the instru- 

 ment. During 72 hours, the record was 4701 miles. 



The assumption given above, that the pressure of the wind 

 acting upon a semi- cylindrical surface is equal to one half that which 

 would be exerted upon a flat surface, having an area equal to 

 that of the diametral plane of the cylinder, is generally 

 accepted as nearly correct by the best authorities, and 

 accords with the recommendation of Rankine in Applied 

 Mechanics. 



In assuming the maximum pressure of the wind, it is 

 considered good practice to accord it a pressure of about 30 



