174 REPORT — 1861. 



reveal the relative directions in which such objects may happen to lie. On 

 both of these points some information has been collected by the recent 

 Commission of Light-houses and Beacons. The amount of this information 

 is, however, remarkably meagre when contrasted with the elaborate details 

 furnished by the portion of the report relative to optical signals. This cir- 

 cumstance is freely admitted ; and at p. xviii of the Report the desirableness 

 of further experiments on the question of sound-signals is distinctly declared. 

 But as the Commissioners received suggestions from several men of science 

 who had paid attention to the phenomena of sound, a condensed sketch of 

 such suggestions will be found to present much of the knowledge we possess 

 upon this question. Before presenting a brief summary of these views, it is 

 right to point out that the earliest experiments which have any important 

 bearing upon the subject were instituted many years ago by M. CoUadon, 

 on the Lake of Geneva. I refer to his well-known researches on the pro- 

 pagation of sound in water. The manner in which the acoustical properties 

 of air are diminished by fogs has recently induced men of science (including 

 many of those who communicated their views to the Commissioners of Light- 

 houses) to recommend the employment of water as a medium for the trans- 

 mission of sound. Almost all we know upon this matter is due to M. Col- 

 ladon*. At first he found that subaqueous sounds were totally reflected at 

 the surface, at such angles as rendered it impossible to hear them above 

 water for distances exceeding 200 metres. To remove this obstacle to his 

 researches he contrived a very ingenious apparatus, that we may for brevity 

 call a hydrophone. Its shape resembled that of a common tobacco-pipe, 

 with a broad and very shallow bowl. Its total length was about 5 metres, 

 or a little more than 16 feet. The pipe was about 18 inches in diameter, 

 tapering at the end close to the ear, where it terminated in an orifice of about 

 8 inches. The mouth of the bowl was closed by a partition, whose surface 

 amounted to a little more than 2 square feet (20 square decimetres). The 

 hydroplione was entirely made of thin sheets of tinned iron. With this ap- 

 paratus M. CoUadon could hear a bell under water at a distance of 14,000 

 metres as well as he could by simply plunging the head at a distance of 200 

 metres. Subsequent to his earlier experiments, M. CoUadon succeeded in 

 transmitting distinctly audible sounds under water to the distance of 35,000 

 metres. The noise of the waves and wind produced little or no effect in 

 diminishing the subaqueous sound, which could be clearly distinguished 

 even when the observer's boat had to be held by several anchors in tempes- 

 tuous weather. The intensity of the sound was so little weakened by di- 

 stance, that M. CoUadon concludes that the decrease is as the simple distance, 

 and not as the square of the distance, as in the air. This is explained by 

 considering that the propagation of sound takes place in a sheet of water, 

 limited between two surfaces, from which vibrations are totally reflected at 

 acute angles. On these grounds, as well as from his experiments, he foresees 

 the possibility of transmitting sounds at sea to distances of some hundreds of 

 thousands of metres, and of applying such sounds to purposes connected with 

 navigation, such as occupy us in the present inquiry. One of his most re- 

 markable results is that of the existence of an acoustic shadow under water. 

 7his was proved by the eflPect of an interposed wall, in experiments made 

 along the shore of the lake. This result is especially important in assisting 

 in determining the direction of a given sound by the interposition of screens, 

 and on this point water seems to possess decided advantages over air. 



* Mem. de I'Inst. Savants Etrangers, v. p. 320. Letter to M. Arago, Antiales de Chimie 

 et de Physique, p. 525, vol. ii. 3« serie. 



