Mr. J. A. Broun on Terrestrial Maynetism. 97 



Is not the zodiacal liglit the magnetic fether in a luminous 

 state, repelled by the solar magnetic poles ? Does not the 

 zodiacal light revolve round the sun ? If so, what is its period of 

 revolution ? Are not the extent and intensity of the zodiacal 

 light related to the periods of the spots, as Cassini and Mairau 

 supposed* ? 



From the known action of the sun on the gases of comets, 

 may we not infer some action of the sun on the gases forming 

 our own atmosphei'e ? 



I shall not enter here into the reasons that I can produce, to 

 show that the diurnal oscillations of the barometer do not 

 depend on the diurnal variations of temperatui'e as their chief 

 cause ; neither can it be explained by M. Dove's ingenious addi- 

 tion of the variation of the pressure of aqueous vapour, which is 

 purely a local phaenomenon. I think I have shown the insuffi- 

 ciency of the theory founded on the combination of these causes, 

 in the Makerstoun Observations : it is insufficient both in Europe 

 and India. AVe require a cause like that of the solar and lunar 

 attraction on the mass of the ocean to produce a double tide 

 daily. May not the facts known exhibit such a cause f? Should 

 not the sun acting as a magnet on the magnetic gases forming 

 our atmosphere, and by induction on the terrestrial magnet, 

 cause the atmosphere to assume an ellipsoidal form, having the 

 greater axis in or near the plane of the equator ; thus deter- 

 mining the greater diurnal oscillation in the equatorial regions. 

 If the form which the atmosphere assumed, under the influence 

 of the terrestrial and solar magnets, were somewhat irregular (as 

 in some of the figures assumed by magnetic liquids between the 

 poles of a magnet, as in M. Pliicker's experiments), we might 

 explain in this way the diminished mean atmospheric pressure 

 near the equator, and the maximum pressure 20° north and 

 south of it. I am not aware that Dr. Faraday or M. Becquerel 

 has determined the specific magnetism of air containing vapour 

 in the form of cloud. Is not the specific magnetism of air di- 

 minished by containing aqueous vapour ? and is not the dimi- 

 nution proportional to the relative humidity of the air, rather 

 than to the absolute amount of watery vapour ? If so, the diur- 

 nal oscillation of the barometer should diminish ceteris paribus 

 with the relative humidity of the air J. 



* It seems to ine that the zodiacal liglit is much brighter and longer this 

 year than I have remarked it during the last five years. This impression, 

 however, is not founded on real measures, as I had made none in the pre- 

 ceding years. 



+ Mr. Joule has found that in magnetizing au iron bar it is lengthened 

 (without ehauge of volume) in the direction of the magnetic axis. 



X I have remarked, in several of the results of the magnetic observations 

 made in Scotland, the coincidences with results for the atmospheric press- 



Phil. Mag. S. 4. Vol. 10. No. 105. Au<j. 1858. II 



