A. MATHEMATICS AND ASTRONOMY. 27 



fact that few observers preserve complete records of the 

 aurora, and that few records which continue over a number 

 of years have a uniform degree of completeness at all times ; 

 having also eliminated the periodic changes, both the diurnal 

 and the annual, and after making allowance for the relative 

 cloudiness of the different stations, Fritz presents a map 

 compiled from the records for upward of two hundred places 

 in Europe, Asia, and America. He discusses with great care 

 the probable value of the observations, and then draws lines 

 of equal frequency of auroral display, or isochasms. 



The zone of maximum frequency is a narrow belt pass- 

 ing just north of the North Cape and the Siberian coast, 

 through the northern part of Spitzbergen, and near Point 

 Barrow, Great Bear Lake and Nain on the coast of Labra- 

 dor. According as we go from this zone northward or south- 

 ward, we come to regions of gradually diminishing frequency 

 and diminishing intensity of auroral display; and it is im- 

 portant to observe that, while south of this zone of maximum 

 frequency the auroral arches are generally seen north of the 

 observer, they appear to stations on the north of it to be 

 south of the observer's zenith, while to those upon the zone 

 they appear indifferently to the north or south. The whole 

 system of belts deduced by Dr. Fritz has very great similar- 

 ity to, and almost coincides with, those given by Muncke in 

 1820, and by Loomis in 1856. The curves of equal frequency 

 generally cut the magnetic meridians at right angles, and ap- 

 parently follow the borders of the continents and the limits 

 of perpetual ice ; upon which fact Professor Fritz seems in- 

 clined to dwell with special attention, and to conclude there- 

 from that the atmospheric electricity produced by the friction 

 of winds blowing over ice-fields has something to do with 

 the exhibition of the aurora. It has been suggested, in car- 

 rying out the same idea, that it may not be unlikely that the 

 aurora varies with the increase and decrease of the ice, 

 whence, by analogy, it may be concluded that the neighbor- 

 hood of the Alps may influence the frequent displays of 

 aurora seen in Northern Italy. 12 A, XL, 14. 



GROUND CURRENTS AND THE AURORA. 



In some remarks on the auroras of February 4, 1871, and 

 1872, Mr. Tarry states that it appears from the observations 



