50 



Anniversary Meeting. 



[Nov. 30, 



phenomenon is in a great measure due to motions among the mole- 

 cules of the residual gas, which themselves become vehicles for the 

 transmission of Electricity through the tube. It is well known that 

 gases at atmospheric pressure offer great resistance to the passage of 

 Electricity ; and that this resistance diminishes (to a certain limit, 

 different for different gases) with the pressure. And the researches 

 in question appear to show that the discharge, manifestly disruptive 

 at the higher pressures, is really also disruptive even at pressures when 

 stratification takes place. The period of these discontinuous dis- 

 charges has not yet been the subject of measurement, but it must, in 

 any case, be extremely rapid. 



The remarkable experiments which have resulted in the liquefaction 

 of the gases hitherto regarded as permanent will be noticed presently 

 when I deliver to their authors the medals they so richly deserve. 



Under the auspices of the Elder Brethren of the Trinity House, 

 and as their scientific adviser, Professor Tyndall has conducted an 

 investigation on the acoustic properties of the atmosphere. The 

 instruments employed included steam whistles, trumpets, steam syrens, 

 and guns. The propagation of sound through fog was proved to 

 depend not upon the suspended aqueous particles, but upon the con- 

 dition of the sustaining air. And as air of great homogeneity is the 

 usual associate of fog, such a medium is often astonishingly transparent 

 to sound. Hail, rain, snow, and ordinary misty weather, were also 

 proved to offer no sensible obstruction to the passage of sound. Every 

 phenomenon observed upon the large scale was afterwards repro- 

 duced experimentally. Clouds, fumes, and artificial showers of rain, 

 hail, and snow were proved quite ineffectual to stop the sound, so long 

 as the air was homogeneous, while the introduction of a couple of 

 burners into a space filled with acoustically transparent air soon 

 rendered it impervious to the waves of sound. As long as the con- 

 tinuity of the air in their interstices was preserved, the sound-waves 

 passed freely through silk, flannel, green baize, even through masses 

 of hard felt half an inch in thickness, the same sound-waves being 

 intercepted by goldbeater's skin. A cambric handkerchief which, 

 when dry, offered no impediment to their passage, when dipped into 

 water became an impassable barrier to the sound-waves. 



Echoes of extraordinary intensity were sent back from non- 

 homogeneous transparent air; while similar echoes were afterwards 

 obtained from the air of the laboratory, rendered non-homogeneous 

 by artificial means. Detached masses of non-homogeneous air often 

 drift through the atmosphere, as clouds pass over the face of the 

 sky. This has been proved by the fluctuations observed with bells 

 having their clappers adjusted mechanically, so as to give a uniform 

 stroke. The fluctuations occur only on certain days ; they occur when 

 care has been taken to perfectly damp the bell between every two sue- 



