316 KEPORT— 1882. 



of the plate the whole amount of which is imperfectly known, and about 

 the distribution of which nothing has yet been ascertained. 



The total observed pressure on a thin plate of small size is somewhere 



in value between 1'4 G A-— and 2 Gr A— - where A is the arear of the plate, 



whicli appears to show (1) that on a small plate the variation of pressure 

 is not very great, and (2) that the negative pressure approaches to at It ast 

 one-half the front pressure. 



The Committee consider that observations might usefully be made to 

 throw light upon some of these points, and they suggest that as with the 

 velocities under consideration, the pressures of air in motion probably 

 follow approximately the laws of the pressure of water in motion, experi- 

 ments should first be made in running water. 



A steady flowing river affords a current of fluid moving with fairly 

 uniform motion at a definite and readily measurable velocity, while the 

 pressures of a stream of water are much more easily measured than those 

 of an air current. Hence the determination of the pressures at diSerent 

 points of the front and back of a plate immersed in a river could be easily 

 made, and would establish the law of distribution applicable to any fluid, 

 to wind as well as water. 



Further, the influence of the form of the obstacle on the pressure 

 produced by the stream could be readily determined, the extent to which 

 one body would be sheltered by another placed at various distances above 

 it could be observed, and the total pressures on such structures as a 

 girder or a pair of girders placed a short distance apart at right angles to 

 the stream could be measured. 



Respecting the law of the variation of pressure with increase of area, 

 the older French experiments give conflicting results, the pressure in 

 some cases being found to increase more rapidly and in others less rapidly 

 than the area, while White, by a comparison of the observed and calcu- 

 lated heel of ships under sail, has arrived at the conclusion that the 

 former is the case. As to experiments on this point we have no recom- 

 mendation to make, bat we believe that the observations now being 

 carried on with a pressure plate of 300 square feet in area by Messrs. 

 Fowler and Baker, at the site of the proposed Forth Bridge, will give 

 much useful information. 



As to the increase of pressure at diSerent elevations above the surface 

 of the ground, which is of special importance in the consideration of 

 engineering structures, we recommend that experiments should be made 

 by placing anemometers at various heights, supported by a lofty chimney 

 or other isolated structure. 



It would appear of the highest importance to engineers that some data 

 should be available to enable them to determine the influence of the 

 exposure of a structure. 



As in the design of marine works the depth of water and the length 

 of fetch enable some estimate to be made of the strength of structure 

 required to withstand the heaviest seas to which they may be exposed, so 

 a careful comparison of the wind pressures observed during the height of a 

 gale at various stations where anemometers exist with any standard within 

 the track of the gale, would enable some estimate of the relative exposure 

 of similar places to be made. 



Exceptionally high gusts of short duration should be excluded from 

 the comparison, as being probably limited in area. 



