WIND PRESSURE. 581 



of slightly over 7iu. diameter supported on the carriage behind it. 

 When the two discs were 4in. apart the rear one was dragged 

 forward or towards the wind with a force of one-fifth of that with 

 which it was driven back when the larger was removed. At less 

 or greater distances than 4in. the effect was reduced. In the 

 second a Sin. disc was experimented upon, a second disc of equal 

 size being placed in front of it. When the two discs were 2in. 

 apart the rear one was unaffected. When the distance was less 

 than 2in. it whs driven forward ; when greater, backward. 



Another curious effect was noticed when a plane surface 

 parallel to the wind was brought in contact with a C3'linder or 

 •sphere so as to stop the escape of the air on one side. In this case 

 the pressure on the cylinder or cone was immediately augmented 

 by about 20 per cent. Thus it appears that the security of a 

 circular fhimney may be impaired by the erection of a building 

 beside it. This effect was not noticed with cubes and rectangular 

 blocks. 



The above experiments were on a small scale, and will not carry 

 as much weight as if they had been on a larger one. Possibly they 

 may at some future time be repeated with more powerful and 

 perfect apparatus. But should they be, it is not, in the author's 

 opinion, likely that any important alteration in the results will 

 take place. Small experiments, if carefully made and honestly 

 reported, do not often give results differing seriously from those 

 obtained on a larger scale. 



The folloAving conclusions, deduced from a consideration of all 

 available dates, are submitted as reliable guides in practice in the 

 southern and south-eastern parts of Australia. 



1. Plane surfaces of not less than 3()0 sq. ft. area are subject 

 to a maximum wind pressure of not more than 201bs. per square 

 foot, and smaller surfaces to a pressure of not more than SOlbs. per 

 square foot in exposed positions. In very sheltered positions half 

 the above values may be taken, and intermediate cases may be 

 dealt with according to judgment. 



2. Square, round and octagonal towers, chimneys, spires, 

 railway carriages, girder bridges and roofs are to be reduced to 

 their equivalent normal plane surfaces by means of the moduli and 

 rules given in the earlier part of this ])aper. 



3. Factors of safety of not less than 2 for stability and 3 for 

 strength should be used. 



In concluding, the author desires to render his thanks to Messrs. 

 Russell and EUery and Sir Charles Todd, the directors of the 

 Observatories at Sydney, Melbourne, and Adelaide respectively, for 

 information as to wind velocities and pressures ; to the physical 

 department of the Melbourne University for loan of models and 

 apparatus ; and to Mr James Mann, the assistant in the engineer- 

 ing laboratory, who arranged the a])paratus and offered many 

 useful suggestions during the course of the experiments. 



