Experiments with Fog Signals 193 



Professor Tyndall described the results of experiments, 

 made off the South Foreland, to ascertain the best means 

 of producing sounds to serve as fog signals at sea. Of the 

 various contrivances tested, the most powerful was a steam 

 siren, with twelve radial slits worked under a pressure of 

 70 Ibs. The experiments had shown the extraordinary 

 extent to which even the most powerful sounds were affected 

 by the state of the atmosphere, the direction of the wind, 

 and the like, but not by falling rain, as had been supposed. 

 A very probable cause of the obstruction of sound, even 

 on a perfectly calm and optically clear atmosphere, was the 

 formation, under a hot sun, of strata of air in different degrees 

 of saturation by aqueous vapour, from the surface of which 

 the sound apparently was partially reflected. In one 

 instance the loudest sounds, on a perfectly calm and clear 

 day, were inaudible at three miles from the shore till after 

 sunset. Also, when the observers came between the shore 

 and the stratified portion of the atmosphere, they noticed 

 the reverberation of the sound from the air to be very loud 

 and distinct. 



Nov. 27th, 238th meeting. In a conversation on Professor 

 Sylvester's communication to the last meeting, Mr. Huggins 

 stated that M. Martin, of the Paris Observatory, had given 

 perfectly flat surfaces to mirrors for heliostats by using 

 convex and concave instead of flat tools, the process being 

 carefully watched and tested from time to time. 



Dec. 1 8th, 239th meeting. Professor Cleik Maxwell 

 exhibited an instrument for applying polarized light to 

 detect the state of strain in a moving viscous fluid. This 

 was placed in a hollow brass cylinder with glass ends, within 

 which another metal cylinder was made to rotate, and 

 parallel to it a polarized beam was passed through the fluid. 

 Liquid Canada balsam showed the effects well, but he had 

 failed to obtain them with gum mucilage or syrup. This 

 method appeared to be capable of indicating the nature of 

 viscosity in different substances. 



1874. Jan. 29th, 240th meeting. Mr. Busk spoke of 

 two skulls of the tiger, recently brought by Mr. R. Swinhoe 

 p.c. N 



