the Value of the Pitot Constant. 353 



The pitot P was fixed on the end of a tube leading along 

 an arm a and passing at ihe centre of rotation through 

 an oil joint o to one limb of a sensitive pressure-gauge. 

 The oil vessel was mounted so that the arm a and the 

 pitot could be rotated in a horizontal plane. The pitot was 

 horizontal and at right angles to the arm and moved round 

 in a circle of 5 feet radius close to a cardboard rim r, 

 the outer surface of which was covered with a series of 

 paper flutings. 



Above a and over the greater part of the apparatus 

 was a horizontal cardboard sheet c to which were attached 

 at equal intervals along radii 16 vertical cardboard structures 

 s ; the pitot passed through holes in these during its 

 rotation. Consistent with free motion of the pitot these 

 holes were made as small as possible. The flutings and 

 screens were effective in breaking up the air-flow in the 

 neighbourhood of the pitot. A horizontal band of card e, 

 3 inches wide was placed round the rim r to stop as 

 much as possible motions under the sheet c from affecting 

 the air outside r. For this reason also muslin curtains were 

 hung from r down to the ground and horizontally across 

 beneath the rotating arm a. The other end of the pressure- 

 gauge was connected to a static pressure, tube, which in this 

 case was a 2-inch cardboard tube z encircling the apparatus 

 and fitted with 16 openings. In this way the static pressure 

 taken was the average of the static pressures around the 

 pitot's path. 



In order to obtain as uniform a motion of the arm as possible, 

 a viscous resistance to the motion was supplied and the arm 

 subjected to a constant turning couple. In the figure, W is 

 a heavy iron wheel from which the metal vanes D were 

 supported ; these vanes dipped into an annular dash-pot 

 containing oil and supplied the necessary friction. 



The device for obtaining a constant couple is shown in 

 fig. 1, b. Each half of the belt from the pulley b passed to 

 ihe turning table d over two fixed pulleys between which 

 was a movable pulley carrying a weight. These two 

 floating weights w l and iv 2 were unequal, and the table was 

 rotated at such a rate as to keep them suspended freely at 

 the same level. When they were balanced in this way the 

 pitot moved at a constant velocity ; the rate of turning could 

 be varied by altering the difference between the weights. 



The pressure-gauge employed was of the Chattock-Fry 

 type used in the work of Stanton and described in his paper 

 (loc. cit,)*. This is a form of tilting U-tube gauge in which 



* An improved form of this gauge but modified so that its water 

 surfaces are trapped by mercury, has also been described in Phil. Mag. 

 [6] xix. p. 451. 



