78 



NIPHER— ELIMINATION OF VELOCITY EFFECTS 



[April 20, 



on a graduated circle. The mouth of the tube could thus be directed 

 towards the head of the train, towards the sky, the rear of the train, 

 towards the ground and then towards the head of the train again. 

 The setting could be made for each ten degrees. The zero angle 

 was taken when the mouth of the tube was directed towards the 

 head of the train, the pressure being then a maximum. The tube 

 had a diameter of one inch, and the walls of the tube were of thin 

 metal. For angles between o and 60°, the pressures were positive. 

 At 60°, the pressure observed was zero. At an angle of 90°, the 

 decrease in pressure was greater than the increase when the angle 

 was zero. 



The values observed are here given, the pressures being in grams 

 per square centimeter. These values may, of course, be represented 

 by an harmonic series. It requires about ten terms. 



These values are plotted in a polar diagram in Fig. i. 



The pressures were measured with a water manometer, having 

 its tube inclined to a slope of one in twenty. The actual readings 

 were, therefore, twenty times the numbers above given. 



These values are the means of twenty measurements. The aver- 

 age velocity of the train as determined by simultaneous measure- 

 ments with another Pitot tube was 41.8 miles per hour. The rela- 

 tive velocity of the air with respect to the train at the point where 

 the measurements were made was 32.5 miles per hour. This lesser 

 relative velocity was due to the fact that air was dragged along with 

 the train. 



It is therefore evident that in such a stream of air, a tube which 

 is to eliminate the velocity effects, must be set with its mouth 



I 



