PHILOSOPHICAL SOCIETY OF WASHINGTON. 43 



As to the result of these observations, it was easy for Dr. Der- 

 ham to conclude that sounds are heard as far in Italy as in Eng- 

 land, when the conditions of the atmosphere are the same ; and 

 these experiments are here cited only for the light they shed on the 

 comparative antiquity of the observation that elevation has an im- 

 portant bearing on the audibility of sounds. 



As to the causes which really affect the intensity of sounds, Dr. 

 Derham seems to have had a very obscure and imperfect notion. 

 His observations under this head are mainly a bundle of contra- 

 dictions, and the causes of these variations he prudently leaves to be 

 investigated by others, seeing, as he says, " that it equally exceeds 

 the grasp of his mind to discover them, and to assign what may be 

 the proper medium or vehicle of sound." He does not, however, 

 fall into the error of measuring the acoustic transparency of the 

 atmosphere by its optic transparency, for he says that the clearest 

 day he can remember, when wind and everything else seemed to 

 concur in promoting the force and velocity of sound, was a day 

 when he could not hear the firing of cannon at a distance easily 

 penetrated by their reports on former occasions. The effect of 

 clear or foggy air on sound, he says, is very uncertain, but as to 

 thick fogs and snow, he affirms that they are certainly powerful 

 dampers of sound, an observation now abundantly proved to be 

 erroneous. 



From some observations made by Gen. Duaue, at Portland, 

 Maine, in 1871, it appears that the fog-signal at that point is often 

 surrounded by a belt of silence, varying from one to one and a half 

 miles in radius. 



From some observations made by Prony, Mathieus, and Arago, at 

 Villejuif, and by Humboldt, Bouvard, and Gay-Lussac, at Mon- 

 tlhery, in France, the two towns being 11.6 miles from each other, 

 it was noticed that while every report of the cannon fired at Mon- 

 tlhery was heard with the greatest distinctness, nearly every report 

 from Villejuif failed to reach Montlhery. The air at the time was 

 calm, with a slight movement of wind from Villejuif toward Mon- 

 tlhery, or "against the direction in which the sound was best 

 heard." These observations were made in 1822. 



In 1872, Prof. Henry observed the same non-reciprocity of sound 

 in approaching the Whitehead fog-signal on the coast of Maine. 

 At a distance of six miles the signal was heard ; at a distance of 

 three miles from the shore the sound of the signal was lost, and was 



