189 



pencil. After this linear distribution of the powder the entire structure 

 was tilted forward slightly about XX', and the line of the powder was made 

 to seek the lowest part of the tubes by gentle tapping. On tilting the 

 frame back to its vertical position, the line of powder was raised slightly 

 along the side of the tubes, and when the glass rod e f g was rubbed at 

 its middle point with a piece of moistened flannel, its longitudinal vibra- 

 tion was communicated to the gas in both tubes, setting up systems of 



r 



X*> 



o 



2i 



B 



U 



fl 



A" 



stationary waves, and causing the powder to fall down at the points of 

 maximum disturbance as shown in Fig. 3. These festoon like figures were 

 sharp and uniform, and capable of accurate measurement, the inter-nodal 

 distances giving the wave lengths of the standing waves within the tubes. 

 Each of the tubes carried near the ends of the horizontal portions, a pair 

 of felt covered brass rings. To the under side of these rings could be 

 quickly attached by means of set screws, the brass meter scale for measur- 

 ing the figures. A sliding sleeve which could be slipped over the tube was 

 provided with a vernier reading to one-tenth mm., which played over the 



