28 Trans. Acad. Sci. of St. Louis. 



Hence the first approximation to the pressure at the axis of 

 rotation is 



P' ^ _ 1 V* 



"5 ~ 2 WW' 



This is numerically equal to the second term in (5). If 

 the length of this rotating tube were r^ 100 cm., the speed 

 being five revolutions per sec, and the temperature were 

 20°C, the value of P' should be — 0.0176 grammes per sq. 

 cm. The pressure actually observed was somewhat over 

 twice this amount as has been previously explained. 



It is evident that if the open end of the L tube were so 

 directed that its axis makes an angle a with the plane of 

 rotation, the compression effect would be decreased. The 

 rarefaction P' at the axis increases, and becomes greatest 

 when the axis is at right angles to the plane of rotation. The 

 current of air then blowing across the open mouth of the tube 

 drags the air out, thus cooperating with the outward tendency 

 due to rotation. A tube of this kind with the L tubes 

 removed and open at the ends, illustrates the action of the 

 ordinary fan blower. 



It was found however, that the vortex motion in the outer 

 air could not be wholly eliminated. The result of varying 

 the direction of the axis of the Pitot tube was however studied 

 by a series of measurements made from the window of a 

 railway car. A tube terminating in an L was thrust out of 

 the car window. The tube could be turned in its mounting, 

 its position being determined by a graduated circle. When 

 the mouth of the tube was directed toward the head of the 

 train the circle read a =0. When the mouth was directed 

 downwards, towards the rear of the train, and upwards, the 

 angle a was respectively 90% 180° and 270° 



The pressure was measured by means of a water mano- 

 meter like that described in a former paper before referred 

 to. The methods described in that paper w^re used. 



The pressures measured for various positions of the Pitot 

 tube are given in grammes per sq. cm. in the following table. 

 The readings which were symmetrical with respect to the 



