352 



Messrs. J. D. Fry and A. M. Tyndall on 



Experimental Details. 

 Centrifugal Metlwd. — One of the chief difficulties of 

 putting this method into practice arises from the fact that 

 the rotating arm inevitably produces a vortex in the air of 

 the laboratory 5 with the result that error is introduced into 

 the observations in two ways : — 



(a) Owing to the motion of the air, the relative 

 speed of pitot and air is not so great as the rate of 

 rotation of the arm would imply. 



(b) The static pressure of the air falls towards the 

 centre of rotation, and the pressure at the open end of 

 the pressure, gauge is not necessarily the same as the 

 static pressure along the path of the pitot. 



It was, therefore, important (a) to bring the air to rest 

 along the path of the pitot, and (b) to connect the open end 

 of the gauge in such a way that the pressure upon it was the 

 average of the static pressures encountered by the pitot. 



Pig. 1. 



(a) 



As far as could be ascertained after many trials, the following 

 arrangement fulfilled these two conditions (fig. 1, a and b). 



