42 Proceedings of the British Association. 



capes to the earth, and rushes along its surface, with the vapor in 

 the lower parts of the atmosphere, towards the equator, and is 

 again carried back by rising vapor to the poles in constant succes- 

 sion, interrupted only in part, by the intense cold of the polar re- 

 gions, causing the air to be then comparatively dry. The least 

 disturbance then, taking place in the highly charged vapor, (either 

 by part of the electricity being drawn off to the earth, or by vapor 

 diffused from the more temperate regions, or by the accession of 

 vapor either more or less charged with electricity,) must cause an 

 instantaneous flash to pass through the whole mass of vapor, by 

 the rush of electricity to restore the equilibrium, thus establishing 

 the Aurora. Magnetism he ascribes to the constant circulation of 

 electricity, and shows that this opinion will account for the lead- 

 ing phenomena, polarity, daily declination, variation, and constant 

 oscillation of the magnetic needle. 



Mr. Espy read a paper to show that the four fluctuations of the 

 barometer which occur daily, are produced entirely by the in- 

 creasing and diminishing elasticity of the air due to increasing 

 and diminishing temperature. When the sun rises, the air be- 

 gins to expand by heat ; this expansion of the air, especially of 

 that near the surface of the earth, lifts the strata of air above, 

 which will produce a reaction, causing the barometer to rise ; 

 and the greatest rise of the barometer will take place when the 

 increase of heat in the lower parts of the atmosphere is the most 

 rapid, probably about 9 or 10 A. M. The barometer, from tliat 

 time, will begin to fall ; and at the moment when the air is part- 

 ing with its heat as fast as it receives it, the barometer will indi- 

 cate the exact weight of the atmosphere. The barometer, how- 

 ever, will continue to descend on account of the diminished ten- 

 sion of the air, and consequent sinking upon itself, as the evening 

 advances ; and its greatest depression will be at the moment of 

 the most rapid diminution of temperature, which will be about 

 4 or 5 o'clock. At this moment the barometer will indicate a 

 less pressure than the true weight of the atmosphere. The whole 

 upper parts of the atmosphere have now acquired a momentum 

 downwards, which will cause the barometer to rise above the 

 mean, as the motion diminishes, which must have taken place 

 some time in the night. This rise will be small, however, com- 

 pared with that at 9 or 10 A. M. As the barometer now stands 

 above the mean, it must necessarily descend to the mean at the 



