158 Mr D. Milne on Earthquake-Shocks felt in Great Britain 



pressure of the atmosphere on the part of the earth's surface 

 affected by them, was undergoing very considerable diminu- 

 tion ; that the barometer had been falling for several hours 

 previously, and that it reached its lowest point very shortly 

 after the principal shock of that night was felt. 



It would have been important to have had the means of ob- 

 taining similar information at the time that the other shocks 

 occurred, on previous occasions in the same month, but from 

 the want of' any barometers in the district then regularly 

 observed, this is impossible. 



It is, however, worthy of remark, that the atmospherical 

 pressure for the year 1839 was considerably lower than usual, 

 and especially in September, as the following results pretty 

 clearly indicate : — 



1^/, In regard to the mean annual pressure, it was rather, 

 though not much, less in 1839, than for several years pre- 

 vious and subsequent. 



2d, In regard to the oscillations of the barometer, as shewn 

 by the mean and maximum ranges in 24 hours, they were 

 greater in the year 1839, than in either the previous or sub- 

 sequent year. 



^d, In regard to the absolute height of the barometer, it 

 sunk lower in the year 1839, than it had done for many years 

 before, or has done since. 



4^/*, In regard to the particular month in which, during the 

 year 1839, the barometer sunk lowest, all of the registers 

 except one agree in pointing out September. 



These observations, as to the amount of atmospherical pres- 

 sure, are important for two reasons. In the first place, they 

 are in accordance with similar observations made in volcanic 

 countries, and especially in South America. In the second 

 place, they are in accordance with the hypothesis, that during 

 the occurrence of earthquake-shocks, there is an unusual quan- 

 tity of electricity passing from the earth into the atmosphere. 

 For it has been very satisfactorily shewn by Mr Snow Harris, 

 that the tendency of an electrified body to give off electricity in- 

 creases, as the pressure of the air upon it diminishes. In order 

 to ascertain this point exactly, he placed in a glass receiver, ca- 

 pable of having the air exhausted from it, two metal balls, — 

 one of them charged with electricity, in order to ascertain 



