'J TO Proceedings of the Asiatic Society. [Sept. 



drawn on the paper only indicates half the total disturbance, i. e. 

 half the difference between the pressure of the atmosphere outside 

 and inside the chinine}*, so that when it is desired to give a diagram 

 showing the true difference, it is necessary to double the vertical 

 scale as has been done in diagram No. 2. 



''This indicator was at work on the 5th August last. At 5 

 minutes past 2 p. m., a small but severe thunder storm passed direct- 

 ly over the Mint, and two remarkable flashes of lightning occur- 

 ed, one following almost immediately after the other. There was 

 loud thunder. 



" When the diagram was examined, it was found that a very 

 marked diminution of pressure had been indicated, as having 

 occurred in the chimney at 2-5 p. m., the time of the thunderstorm. 



" The diagram, with an enlargement to shew in tenths of an inch 

 the actual amount of disturbance, is given. It will be seen that 

 two distinct falls in pressure in the chimney are indicated, and that 

 after the disturbance had ceased, the pressure within the chimney 

 remained somewhat less than it was before the storm. 



" The height of the chimney is 80 feet and the inside diameter 5 

 feet." 



The President said that, assuming the phenomenon described by 

 Col. Hyde to be attributable to the passage of the thunder storm, 

 one explanation only occurred to him. The storm was distin- 

 guished by a thunder clap, which caused the windows of houses to 

 rattle. He thought it possible that the concussion thus apparently 

 produced by the impact of the sound wave might have a very 

 appreciable effect upon the barometrical column. Ool. Hyde's 

 apparatus measured the difference between the simultaneous atmo- 

 spheric pressures on the two legs of the indicator, inside the chimney 

 and outside the chimney, respectively, and it was obvious that the 

 sound wave would travel more slowly through the heated and rarificd 

 air of the chimney than through the outside air : therefore the outside 

 leg would be affected by the concussion before the inside leg. He 

 pointed out that if this happened, the consequence would be just 

 such a disturbance in the uniformity of the diagram drawn by the 

 self-acting register, as Col. Hyde had exhibited. He had not the 

 data relative to the actual pressures, temperatures, densities, ami 



