NO. 4 



WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 



35 



measurement (below 30 miles per hour) the screw 5^ is turned until 

 the level L^ shows horizontal. The tube is then inclined 3 degrees. 

 The instrument is thus quite independent of the leveling of the table 

 or bench on which it may be used. Connection between the reservoir 

 and glass tube is made by a short piece of rubber tube. Displace- 

 ment of the liquid in T is read on a scale of 600 half millimeters 

 attached to the frame. This manometer was made by a skilful instru- 

 ment maker, and great care was taken to set the spirit levels at the 

 correct angles. The best grade of German glass tubing was used, and 

 each tube was carefully cleaned with strong sulphuric acid and potas- 

 sium bichromate. 



If there are no appreciable errors in the leveling, the correct head 

 of liquid (alcohol, 95 per cent, stained red with fuchsine dye) is 



Fig. 8. — Alcoliol manometer. 



given from the geometrical construction. Thus : Head of liquid = 

 displacement in Txsine of inclination. A small correction can be 

 made for the depression of the liquid in R as the level in T rises. 

 This also can be computed from the dimensions. 



The density of the alcohol was taken on a chemist's " West fall 

 Balance " to a precision of o.i per cent. The effect of surface tension 

 is to cause the level in T to be slightly higher than the level in R 

 when the two ends of the manometer are under the same pressure. 

 This is not an error in the instrument, since the zero setting takes 

 account of it. 



Tests were made by connecting both the reservoir end of the 

 alcohol gage and one leg of the Chattock gage to the same static 

 pressure made by a water column. In this way errors due to fluctu- 

 ations of pressure were eliminated. The Chattock gage readings 

 were taken as a standard for reference. The same Chattock gage 



