B. TERKESTRIAL PHYSICS AND METEOROLOGY. 133 



All the phenomena that he has observed and described in 

 regard to the ilhiminated edges of clouds show very plainly 

 that in these cases the polar light was produced in the region 

 of the clouds, and even lower. We knoAV by numerous ob- 

 servations that the number of thunder and lightning storms 

 diminishes considerably as we approach the polar regions, so 

 that they no longer occur in the latitude of 70. Must we 

 then conclude that in these regions the clouds are completely 

 deprived of electricity ? Certainly not ; but only that the 

 electrical discharges are made in some other way. In these 

 high latitudes, electricity is discharged not only by clouds, 

 but also directly by damp air, as takes place in the winter in 

 the temperate zones. A great many direct observations 

 prove the existence of slow discharges of this nature ; and a 

 very remarkable confirmation is given by Angstrom, Avho on 

 one occasion proved the presence in the spectrum of the 

 yellow or auroral ray over almost the entire sky. /S/?^^^/i- 

 so?iia?i Hep. ^181 4, 232. 



KELATION OF THE PHASES OF THE MOON TO ATMOSPHERIC 



PKESSURE. 



In a memoir by Ludicke, he shows that the atmospheric 

 pressure diminishes with the waxing and increases with the 

 waning moon. The pressure is less at the perigee than at 

 the apogee, and in general the effect of the moon upon the 

 atmosphere is the inverse of that which it produces upon the 

 ocean. The observations on which his results are based ex- 

 tend over eight yeais; but the actual effect of the moon 

 upon the barometric pressure, although decided, is yet ex- 

 ceedingly small. 



IRON IN ATMOSPHERIC DUST. 



As the result of an examination of atmospheric dust, the 

 mineral residuum from the melting of snow, etc., with special 

 reference to iron in the atmosphere, Mr. Young remarks that 

 this metal, in appreciable quantity, occurs in the dust accu- 

 mulated in old buildings during a long period of time, and is 

 usually of a globular form, showing that it has been formed 

 at a high temperature. The iron as found in melted snow is 

 much more irregular in shape, and is more abundant in pro- 

 portion when the snow is collected at the lower levels. Mr. 



