410 Prof. A. P. Chattock on the Velocity and 



observation ; and after three or four unsuccessful attempts 

 the following arrangement was hit upon. 



The lettering in fig. 3 corresponds with that in fig. 2. The 



Fiar. 3. 



\Z£/?OOF Z) 



T, 



discharging point B is supported in a narrow sliding glass 

 tube, drawn out at the end B ; the plate A is replaced by a 

 ring of smooth metal : and the ebonite box by the wide glass 

 tube E, of which the ends are connected to the two limbs of 

 the U-tube pressure-gange by the side tubes T x T 2 . The ring- 

 is also supported by a narrow glass tube through which 

 passes a wire connecting it with the galvanometer, and which 

 is itself the prolongation of a micrometer-screw. By this 

 screw the ring can be moved along the tube, and the values 

 of z can thus be altered and measured in terms of the screw- 

 pitch. 



If p is the difference of pressure in dynes per square centi- 

 metre measured on the gauge, and a the cross-section of E, 



P=pa+p f , 



where P means the same as before, and p is that part of the 

 wind- pressure which is stopped by the ring ; it being assumed 

 that the cross-section of E is not appreciably reduced by the 

 presence of A and B and their supports ; and that T x and T 2 

 are so far apart that the momentum of the wind has all been 

 converted into pressure where they occur. 



This quantity// is one of the drawbacks of the new method. 

 It is necessarily unknown ; and it was partly in order to 

 reduce its value to a minimum that a ring rather than wire 

 gauze or perforated metal sheet was finally adopted to replace 

 the original plate, A ; the idea being that the wind would pass 

 through the ring and eddy back outside between it and the 

 tube E without much opposition, p' can never vanish, how- 

 ever, unless the ring is infinitely thin, as the pressure pro- 

 duced by the ions just before they finish their course is 



