[LOUDON] A CENTURY OF PROGRESS IN ACOUSTICS 49 
gestion of Laplace, by members of the Bureau des Longitudes, to 
determine the velocity in air and other media. These experiments, 
which were the beginning of truly scientific work in this subject, were 
performed by Prony, Arago, Mathieu, A. de Humboldt, Gay-Lussac and 
Bouvard, between Montlhéry and Villejuif, cannon being fired at both 
stations, The result obtained was 331 m. at zero temperature, with an 
increase of 0.6 m. for each degree above zero. In the course of these 
experiments it was observed that the cannon fired at Villejuif were 
all distinctly heard at Montlhéry, whilst the reciprocal reports were 
so faint that only a small number were heard. Tyndall long after- 
wards, in 1875, explained this curious phenomenon, attributing it to the 
existence at Villejuif of a heterogeneous atmosphere, caused by the 
heated air which came from Paris. 
Since the memorable experiments of the Bureau des Longitudes of 
Paris, various individuals have from time to time undertaken to solve 
the same problem. Among these may be mentioned Moll and van 
Beck (at Utrecht), Gregory Woolwich, Stone and Captain Perry in 
voyages to the polar regions in 1822, 1824, and Kendall in the 
Franklin expedition in 1825. In some of these experiments the 
temperatures ranged from 2° to—40°, the results obtained according 
with the theoretical values, In 1823 Stampfer and Myrback conducted 
experiments between two stations in the Tyrol at a difference of level 
of 1364 m.; a similar experiment being undertaken in 1844 in Swit- 
zerland by Bravais and Martin with a difference of level of 2079 m. 
Both experiments confirmed the law that the velocity of sound in air 
is independent of the pressure. 
In all these experiments the exactness of the results was affected 
by the difficulty of estimating accurately the time between the percep- 
tion of the flash and that of the report. Different observers of course 
gave different estimates. ‘his source of error was first eliminated by 
Victor Regnault, who in his long series of researches between 1860 
and 1870 made use of the graphical method, electric signals being 
employed to measure time intervals. Regnault’s experiments were 
conducted in ? tubes (part of the Paris sewers) varying in length from 
70 m. to 4900 m., and of diameters from 0.11 m. to 1:10 m. Experi- 
ments were also conducted in the open air by means of reciprocal shots 
fired from two stations at a distance of 2445 metres. The number of 
the shots fired was 334. These researches of Regnault represent such 
an enormous amount of work that I shall attempt to give only the 
principal conclusions deducible from them : 
1. In a cylindrical tube the intensity of the wave varies, dimi- 
nishing with the distance. The narrower the tube, the more rapid is 
the diminution. 
Sec. TI, 1901. 4: 
