ON PRESSURE MEASUREMENTS IN A FLUID 



STREAM.* „^^^^^, 



Francis E. Nipher. new YOk 



BOTANICAL 



The earlier measurements of wind pressure were made with ^^aRDE-n. 

 pressure boards, in which the total force against the boards 

 was measured by means of springs. In some cases the board 

 was moved through the air on a rotating arm. Similar ex- 

 periments have been made with the board immersed in water. 



In all of this work, the information is of little or no value 

 in the study of wind pressures over buildings or other struc- 

 tures. For example, in case of wind pressures in the angle of 

 a building where the wind is practically checked, a pressure 

 board would show no result. The pressures on the front and 

 back sides of the board would be practically equal. And the 

 distribution of pressures over such a board in free air is 

 wholly different from that over any building when exposed in 

 the same current of air. 



The Pitot tube has also been used for wind pressure de- 

 terminations. In this device the rarefaction which exists on 

 the leeward side of the pressure-board is wholly eliminated. 

 The results obtained by the Pitot tube are therefore very 

 different from those obtained from pressure boards. 



When a Pitot tube is placed with its open end exposed 

 normally to an air current, the pressure thereby developed 

 within the tube is that which would result from the slowins: 

 down of the wind to zero velocity. It is the pressure that 

 would develop in the angle of a building, if still air in con- 

 tact with the building could not escape over the roof, but 

 were under compression by the stream of air surging against 

 it. Such a consideration is not in general realized on the 

 surface of a building. The air under pressure is not wholly 

 checked. 



* Presented by title to The Academy of Science of St. Louis, May 7, 1906. 



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