258 PROGRESS OF METEOROLOGY IN 1889. 



ation agreed in direction and phase with the calculation, ou the as- 

 sumption that the seat of the force is outside the earth. In the present 

 more complete investigation the matter has been more fully taken up 

 and the original conclusions have been confirmed. 



The observations taken at Bombay,Lisbou, Greenwich, and St. Peters- 

 burgh are used, and the potential is computed in thirty-eight terms 

 of a series of surface harmonics by means of the horizontal compo- 

 nents only. From the potential thus computed the vertical force is de- 

 duced both on the assumption of an inside and an outside origin of the 

 variation. By tabulating the am^Dlitude and phase of the forces com- 

 puted on each assumption and by comparing the results with the act- 

 ually observed values a complete disagreement is found with the results 

 obtained on the assumption that the disturbing force is inside the earth 

 and nearly complete agreement on the alternative hypothesis. 



The observed amplitudes are found in all cases to be considerably 

 smaller than the computed ones. 



In an appendix Prof. H. Lamb shows that if the earth be heated as a 

 conducting sphere, in which induced currents are excited by an exter- 

 nal cause, this reduction in amplitude may be accounted for. 



Prof. Balfour Stewart's suggestion that convective currents in the 

 atmosphere moving across the lines of the earth's magnetic forces are 

 the causes of the daily variation, gains much in probability by this in- 

 vestigation. If the daily variation of the barometer is accompanied by 

 a horizontal current in the atmosphere similar to the tangential motion 

 in waves propagated in shallow canals, and if the conductivity of the 

 air is suflicieutly good, the effects on the magnetic needles would be 

 very simihxr to those actually observed. {Nature, xxxix, p. G22.) 



Auroras. — Mr. H. Hildebransson gives a summary of the result of 

 the elaborate observations of the aurora made by the Swedish polar 

 expedition at Bossekop (situated in the maximum zone of auroras, on 

 the coast of northern Norway.) 



(1) A mean of 371 measures gave the azimuth of the summit of the 

 auroral arch in S. 24° 12' E. 



(2) A mean of 87 measures on the position of the center of the corona 

 gave its altitude 79° 55', azimuth S. 7° 12' E. This point is nearly in the 

 magnetic zenith, but not in the same vertical as the highest point of the 

 arch. 



(3) The breadth of the auroral arches varies with their elevation 

 above the horizon. The arches consist of rays running in the direction 

 of the breadth of the arch and converging toward the magnetic zenith. 



(4) The auroral light sometimes formed a true spherical zone parallel 

 with the earth's surface, thus floating in space as a horizontal layer of 

 light. 



(5) The movements of the arches from north to south and from south 

 to north were almost equally frequent. 



(6) Anomalous forms of arches were very frequent. 



