Propagation of JVaves in Gaseous Media. 169 



which the gas is submitted. The only direct experiments which 

 can be here cited in confirmation of this law were made in the 

 open air. They are those of HH. Stampfer and Myrbach in 

 1822 in Tyrol, between two stations having a difference of level 

 of 1364 metres, and those of MM. Bravais and Martens, per- 

 formed in Switzerland in 1844, at two stations whose difference 

 of level was 2079 metres. Unfortunately the mean barometric 

 pressures which the air suffers at the two places does not differ 

 sufficiently from the pressure at the level of the sea. 



I have, in my Memoir, given two series of experiments to de- 

 termine the velocity of propagation of sound in the air when 

 under various pressures, and contained in tubes of 0' 108 metre 

 diameter. 



The first experiment was made with the gas-pipe of the route 

 militaire near Ivry, having the length of 567*4 metres. The 

 pressures varied, from 0*557 to 0*838, and the consequent densi- 

 ties of the air from 1*0 to 1*5. 



The second experiment was made in a little horizontal tube in 

 the court of the College de France, the length of the tube being- 

 only 70*5 metres. The pressures varied from 0*247 to 1*267; 

 consequently the density of the air varied from 1 to almost 5. 



It was not possible to establish any sensible difference in the 

 rate of propagation of sound in air under these very different 

 pressures. My experiments accordingly confirm the exactness 

 of the law which I have just enunciated. 



VII. If we compare V and V 2 (the velocities of the same wave in 

 two different gases having the same temperature and being under 

 the same pressure), and if further we assume that they obey 

 Mariotte's law, that they have the same coefficient of dilatation, 

 that they satisfy Poisson's law, &c, in short that they are per- 

 fect gaseous media, we ought, according to theory, to have* 



V'_ Id 



v-Vj 



So that, if one of the gases is atmospheric air, and if 8 represents 

 the density of the other gas compared with air, we have 



Hitherto no direct experiment has been made to determine 

 the rate of propagation of a wave in any other gas than atmo- 

 spheric air. Attempts have only been made to prove the exact- 

 ness of the above law in an indirect way, founded on the theory 

 of sonorous tubes. 



* d and d' being the respective densities of the gases. — F. G. 



