C. GENERAL PHYSICS. 159 



ward that of the square root of the diameter of the drops. 

 It thus appears that, for any note of waves of sound, there 

 is a certain size of drop with which a fog will produce the 

 greatest effects. 12 A, IX., 216. 



YIBRATIOXS OF LIQUID SURFACES. 



The undulations which are produced upon liquid surfaces 

 when a heavy tuning-fork is brought into contact with the 

 vessel containing the liquid have been investigated by Bar- 

 thelemy. He observed the phenomena by allowing a beam 

 of sunlight reflected from the liquid surface to fall upon a 

 screen. Barthelemy deduces the two following laws for 

 rectangular vessels: First, that the breadth of the undula- 

 tions is inversely as the number of vibrations; and, second, 

 that the distance between two lines produced by the same 

 tuning-fork is independent of the density of the liquid. The 

 author calls attention to the general resemblance of these 

 wave surfaces in the basin of a fountain to the waves of the 

 sea; even in the sand on the sea-beach they may be recog- 

 nized ; so also clouds are arranged often in parallel bands, 

 and an ingenious application of these facts accounts for the 

 phenomena of stratification produced by electric discharges 

 in rarefied media. 4 D, VII., 1874, 590. 



ACOUSTIC TRAXSFAEENCY OF THE ATMOSPHERE. 



Professor Tyndall, as the ofticial adviser of the older breth- 

 ren of the Trinity House, has under their auspices investigated 

 the audibility at great distances of fog-sicmals at the stations 

 Dover and South Foreland. Fog-horns, fog-whistles, and the 

 siren were employed to give the signals, each being worked 

 by a steam-engine. Artillery discharges were also employed 

 as signals. The variation in the audibility of the sounds at 

 distances varying from one half of a mile to three miles was 

 very remarkable. A sudden acoustic darkness would seem 

 at times to settle upon the atmosphere. Days that were 

 perfectly clear optically were sometimes the most impenetra- 

 ble to sounds. Professor Tyndall remarks that there may 

 be states of the atmosphere associated with rain that are un- 

 favorable to sound, but to rain itself he has never been able 

 to trace the slightest deadening effect. On certain days of 

 foggy weather, both over the ocean and also in the city of 



