28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



If one end of an open tube be pointed into a stream of air and the 

 Other end be attached to a manometer, the total dynamic pressure will 

 be recorded. On the other hand, if a tube with closed end be pointed 

 into the wind and further fitted with a conical or parabolic tip, the 

 stream line is only slightly deflected and distorted. If then small 

 holes or slots be cut in this tube at a distance well back from the tip, 

 the wind should blow past these openings and the interior of the tube 

 should be subjected to the static pressure of the stream. This pres- 

 sure can be measured by connecting the tube to a manometer. 



It is generally accepted from the results of tests that any open- 

 ended tube of any size, if pointed fairly into the wind, will correctly 

 transmit the dynamic pressure. 



It is equally common knowledge that the correct transmission of 

 the static pressure is not so simple. Widely different values are 

 obtained with dift'erent forms of tube and static orifice, and many 

 tubes, such as the Dines and the Recknagel, must be calibrated against 

 some standard. It is obvious that the nose of the tube should be of 

 easy form, that the tube should not be large in diameter, and that it 

 should be carefully polished in order that the air stream may pass 

 undisturbed. The best form of entrance will introduce some disturb- 

 ance, so that such static openings as are used should be placed well 

 back from the nose on the cylindrical portion. The form and size of 

 the openings will be discussed later. 



The Pitot tube may consist of two separate tubes or a double tube 

 made up of a pair of concentric tubes, the dynamic tube being enclosed 

 within the static tube. Since the dynamic tube transmits the pressure 



p+ f — =po, and the static tube transmits p, it is sufficient to connect 



the two tubes to the two ends of a U-tube filled with liquid. The 

 reading of the instrument is then proportional to the difference 



between the pressures transmitted and hence to - — . Knowing the 



2g 



density, the velocity may be computed. The density of air depends 

 on the pressure, temperature, and humidity. Avoidance of the 

 necessity for calculating the density for ordinary aerodynamical tests 

 would be of great assistance. 



Elimination of Density of Air 



It is generally accepted that the forces produced by a fluid in 

 motion with reference to any solid object depend on the size, shape, 

 and attitude of that object, the velocity, the density of the fluid, and 



