310 METEOROLOGY AND ALLIED SUBJECTS, 



of the records since 1763, finds the values of the annual variations of 

 barometric pressure at Prague, Milan, Vienna, and Munich, are satisfac- 

 torily presented by the assumption that the longer period in these varia 

 tions agrees with the longer period of the auroras, and these phenomena 

 attain their maxima and minima at the same time ; on the other hand, 

 F. Chambers, from the barometer records at Bombay, arrives at the 

 opposite result, namely, that the maxima of auroras correspond with the 

 minima of barometers ; but the discrepancy between these results may 

 be only apparent when once we are able to properly ai^preciate the facts 

 deduced by Forssman (Upsala, 1873), who arrived at the following re- 

 sults : 



(a) Definite variations in barometric pressure are observed during 

 strong magnetic disturbances or auroras that extend beyond their 

 appropriate zones. 



(&) The barometer variations have opposite signs in different portions 

 of Euroi)e, and probably also America. 



(c) The limit between the regions of oj^posite signs is determined, at 

 least in Europe, by a line that begins north of Scotland and passes 

 southeastward through the Black Sea. 



{d) During the presence of auroras and magnetic disturbances the 

 barometer rises, or is at its maximum, in the region northeast of this 

 line, and sinks, or is at a minimum, in a region southwest of this line. 



(e) In the southwestern region the barometer is either rising or fall 

 ing according as the magnetic horizontal component is very large or 

 small ; the opposite is probably true in the northeastern portion. 



If Forssman's conclusions are confirmed by future investigations, 

 then it will appear that the relation between auroras and the weather 

 is far more complicated than has been hitherto assumed. 



(19) The relation between aurora and polar bands has long been believed 

 to be quite definite. These relations show themselves not only in the fre- 

 quent simultaneous ai^pearances of the two phenomena, but also in the 

 frequent auroral form of the clouds known as polar bands, which, indeed, 

 allow one to conclude the action of the polar force within them simply 

 by reason of their peculiar arrangement as parallel streaks and regu- 

 larly broken or stratified groups. There is here an undeniable relation, 

 either direct or indirect. Among the numerous relations recorded by 

 various observers quoted by Fritz, we cite the following: Humboldt ob- 

 served that the vanishing-point of the polar bands moves gradually from 

 east to west. Cramer states that the auroras change themselves into 

 clouds, the whole heavens being covered with clouds if the phenomena 

 lasts for a sufficiently long time. At Upsala, February 4, 1874, a ring 

 of green color was observed around the moon during a fine aurora. 

 From the observations at Bossekop, Bravais deduced the mean direction 

 of the cirro-cumuli E. 28^.3 N., while for the auroral arcs it was E. 

 210.6 ]!T. Stevenson, at Dunse, in Scotland, found the annual fre- 

 quency of cirrus clouds to run parallel with the frequency of the auro- 

 ras. Winnecke, from observations at Poulkova, concludes that the cirrus 



