18UO.] On Barometric Oscillations during Thunderstorms. 59 



There was some loss in Series I and II in removing the acid from 

 above the stopcock ; this was done by aspirating air through the tube, 

 which was heated by a Bunsen's burner, and absorbing the acid in a 

 soda solution ; but an acid fume was formed, which refused to be 

 absorbed. In Series III, the apparatus with the jacketed stopcock 

 was used, and the acid which collected above the stopcock was 

 washed out. The vapour-density in the extreme right-hand column 

 is calculated for a mixture of hydrochloric acid and water in the pro- 

 portions found by titration. These results show that no compound of 

 acid and water is formed, at least at the temperature employed ; they 

 confirm the results of Bineau, who found a vapour-density 10*04 at 

 the atmospheric pressure (' Annales de Chimie,' vol. 7, 1843, p. 257). 



III. " On Barometric Oscillations during Thunderstorms, and on 

 the Brontometer, an Instrument designed to facilitate their 

 Study." By G. J. SYMONS, F.R.S. Received April 24, 

 1890. 



The fact that a rise of the barometer occurs during thunderstorms 

 has been supposed by many to be newly discovered through the 

 general establishment of self-recording barometers ; but Dr. Hellmann 

 has shown that it was noticed by J. J. Planer as far back as 1782. 

 In 1784, Rosenthal epitomised the facts as follows: "When a 

 thunderstorm approaches the place where a barometer is situated, 

 the mercury in the tube begins to rise ; the nearer the thunder-cloud 

 comes to the zenith of the observer, the higher does the mercury rise, 

 and it reaches its highest point when the storm is at the least distance 

 from the observer. As soon, however, as the cloud has passed the 

 zenith, or has become more distant from the observer, the weight of 

 the atmosphere begins to decrease and the mercury to fall." A few 

 years later, Toaldo determined the amount of the rise in several 

 storms, and found it to be between 1 and 2 lines (0'09 in. to O18 in.). 



Professor Strehlke (in 1827-30) made several sets of observations, 

 and found the rise to be from 0'04 in. to 0'06 in., and was probably 

 the first to point out that the highest point of the barometer is not 

 absolutely synchronous with the passage of the centre of the storm- 

 cloud, but seems rather to be always at a certain distance from it. 



Kaemtz, in his ' Lehrbnch ' (1832), suggests that the rise is pro- 

 duced by the inrush of air towards the site of the storm, this accu- 

 mulation causing the rise of the barometer as the storm nears the 

 zenith. 



Although Luke Howard had a recording barometer at work in the 

 early part of this century, he seems to have failed to notice the 



