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 
foo tooo rr a 
A 
X x' 
ie. 2 
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 
