70 
THE TELEGRAPH. 
tubes, by noting very exactly the time taken by a sonorous 
undulation to traverse the tube as far as the obstacle. 
The arrangement is shown in Fig. 32. A strip of slightly 
vulcanized india-rubber, a , b, is selected of about *013 inch in 
thickness when it is not stretched, and weighing about 4 grains 
per square inch. This membrane is stretched between pairs of 
metallic clips, connected by screws passing through the india- 
rubber. To the centre of the latter a small metal disc, c, is 
attached, and above this is a pointed screw, cl An electric cir¬ 
cuit is formed when by the movement of the india-rubber, c 
and d come into contact. 
This elastic membrane is placed over the open end of the tube, 
and the time of any rising up of the india-rubber is registered 
by means of electricity on a revolving cylinder. A sound¬ 
wave is produced in the tube by firing a pistol as shown in 
Fig. 32. This wave travel's in the tube with the velocity of 
1,074 feet per second, and striking the obstacle, is there 
reflected and returns in the opposite direction, causing a rising 
of the india-rubber on its arrival. Thus a mark is made on 
the cylinder. The membrane in its turn again reflects the wave 
towards the obstacle which sends it back anew, and a second 
mark is made on the cylinder. If the interval of time elapsing 
