398 Intelligence and Miscellaneous Articles. 



have been forced to use it for my own convenience ; and the benefit 

 I have experienced from its use has induced me to seek its publica- 

 tion in your Journal. 



I am, Gentlemen, 



Your obedient Servant, 



W. H. Preece. 



ON THE VELOCITY WITH WHICH A DISTURBANCE, PRODUCED 

 IN A GASEOUS MASS CONTAINED IN A CYLINDRICAL TUBE, IS 

 PROPAGATED. BY M. P. P. LE ROUX. 



The memoir which I have the honour of submitting to the 

 Academy contains the description and the results of experiments 

 made in 1862 and 1863, on which I have already made a commu- 

 nication, and of which several members of the Academy have been 

 witnesses. 



I proposed to solve experimentally the following question : — to 

 measure directly by purely mechanical means on a relatively short 

 base the velocity of propagation of a single impulse, communicated 

 to a gaseous mass of determinate temperature contained in a 

 cylindrical tube. 



I began by devising a new chronoscope, based on the law of the 

 descent of bodies, by which short intervals of time may be measured 

 with great accuracy, and which is always ready to work in exactly 

 the same manner without needing special preparation — an advan- 

 tage which chronoscopes moved by clockwork do not possess. As 

 a means of pointing, I used the action of the induction spark on a 

 slightly iodized surface of silver — a method on which I shall not 

 dwell, as it has already been the subject of a communication to the 

 Academy. 



The tube in which the disturbance was to be produced was 7 

 centimetres in diameter by 72 metres in length ; it was bent in 

 two, and it could be surrounded with water or with melting ice. 



The two ends of the tube were closed by very thin and tightly 

 stretched membranes of vulcanized caoutchouc. A wooden hammer, 

 moved by springs, struck one of these membranes with a single blow, 

 and thus imparted to the air in the pipe a disturbance which in 

 about y of a second moved the second membrane. In front of each 

 was placed a kind of little pendulum traversed by the induction 

 current of a Ruhmkorff's coil ; the motion of the membrane dis- 

 turbing the pendulum broke the secondary current, and produced 

 the induction spark, the trace of which was received on the scale of 

 the moving chronoscope. In this way the beginning and end of the 

 propagation of the impulse were indicated. 



This method, which is very simple in principle , requires in practice 

 a large number of precautions, which cannot be given here. 



To dry the air in the tube, a circulating motion was imparted to 

 it by means of a special pump, so as to pass it several times over 

 substances which would deprive it of moisture and carbonic acid. 

 As an accessory, and also in order to estimate the tension of the 

 small quantity of aqueous vapour which might still exist in the air, 



