ON THE BEST MEANS OF ASCERTAINING WIND PRESSURES. 315 



Report of the Committee, consisting of Mr. James Glaisher, Mr. C. 

 W. Merrifield, Sir Frederick Bramvvell, Professor 0. Eeynolds, 

 Professor W. Cawthorne Unwin, Mr. Eogers Field, Mr. T. Hawks- 

 ley, and Mr. A. T. Atchison {Secretary), appointed to consider 

 and report upon the best Tiieans of ascertaining the effective Wind 

 Pressitres to ^vhich buildings and structures are exposed. 



The total pressure on small plane surfaces due to actual winds in high 

 and exposed positions has to a great extent been ascertained with suffi- 

 cient accuracy for engineering purposes, and the relation between the 

 pressure and the velocity on flat plates is also known with accuracy 

 enough to make the observations with velocity anemometers available in 

 estimating the probable maximum pressure in such exposed positions as 

 are chosen for the placing of anemometers. 



It must be assumed for the present at least, that for engineering 

 purposes, pressures of 80 lbs. or even 90 lbs. per square foot on small 

 surfaces have been correctly recorded in extremely exposed positions 

 such as the Bidston Observatory, while pressures approaching the limit 

 prescribed by the Board of Trade (56 lbs. per square foot), may possibly 

 act on engineering structures in exposed positions. 



In applying these data to engineering structures the question arises 

 whether their form, size, and exposure is so difiFerent from a thin ane- 

 mometer plate that the effective pressures to which they are subject can- 

 not be deduced from the latter. 



To answer this question inquiries on the following points appear 

 necessary. 



i. The law of the distribution of the pressure of a fluid impinging on a 

 surface. 



ii. What is the relation of the direct front pressure and the negative 

 pressure produced by the foi'mation of a partial vacuum behind the plate 

 due to the gradual curvature of the fluid stream lines in passing an 

 obstacle. 



iii. What is the law of variation of pressure with increase of area o£ 

 surface. 



iv. What is the law of variation of pressure with increase of elevation 

 above the ground surface. 



V. How far the form of actual engineering structures diminishes the 

 front or back pressure which would be experienced by a flat plate similarly 

 placed. 



There seems reason to believe that the maximum direct pressure on a 



Gv^ * 



surface due to fluid impinging on it at a velocity v is -— when G is the 



weight of a cubic unit of the fluid. From the point where the pressure 

 is greatest the intensity must diminish towards the. edges, where it may 

 possibly even become negative (or less than the barometric pressure), 

 but the law of variation is almost entirely unknown, and requires investi- 

 gation by direct experiment. 



Further, the total pressure registered by an anemometer plate is made 

 up of the direct pressure which, from what has been said above, cannot 



exceed the area multiplied by G— and a negative pressure at the back 



