CURRENTS
7
ating and not a uniform and steady pressure. In the Report of the Committee on Wind Pressure on Railway Structures (1881), the following formula is adopted to indicate the relation between wind velocity and pressure:—
V being miles per hour, P maximum pressure, the lowest value of V being 40 miles per hour. The results obtained by this formula between 40-80 miles per hour show a close approximation with experimental observation. Thus, with a wind having a maximum hourly run of 80 miles, the maximum pressure by formula is 64 lb. per square foot; according to experiment, 65.5 lb. per square foot.
   Currents.—Current scour may be either the slave or the tyrant of the designer of harbour and river works. Dr. J. S. Owens, in a paper entitled “Experiments on the Transporting Power of Sea Currents”,1 evolved the following formula:—
       If d — diameter of a flint stone ip inches,
V	= velocity required to move it in feet per second.
                             V2
      For ordinary pebbles d = — approximately.
   Fine sand -¿V to inch diameter begins to move under a current of 0.60 to 0.80 foot per second.
1Journal of the Royal Geographical Society, April, 1908, p. 417.