METEOROLOGY. 551 



sun-spots. For the annual period lie finds the following numbers, which, 

 however, give but slight indications of the ordinary maxima at the 

 equinoxes : 



January 84 July 91 



February 77 August 72 



March 88 September 95 



April .... 92 October 85 



May 87 November 82 



June 90 December 74 



As regards the daily period, he finds a confirmation of the law an- 

 nounced by him in 1881, namely, that there is an annual variation in the 

 time of daily maximum by reason of which the hour of maximum in 

 the winter half of the year is decidedly earlier than the summer half 

 and the mean duration is greatest at the time of the equinoxes. (Z. 0. 

 G. M., xvn, p. 417.) 



H. J. Groneman, of Groniugen, combats the statement of Sophus 

 Tromholt, that in many cases the aurora is au apparently local phe- 

 nomenon, and that it often occurs at slight altitudes above the earth's 

 surface. He examines in detail the observations quoted by Tromholt, &c, 

 and maintains, after minute analysis, that they give no ground for such 

 conclusions. (Z. 0. G. M., xvn, p. 187.) [The present writer announced 

 precisely the same conclusions in the Eeport of the Chief Signal Officer, 

 1S7G, p. 311, as based on the study of many auroras, especially that of 

 April 7, 1874, and is still inclined to sustain the views of Tromholt.] 



Sophus Tromholt replies to Groneman's criticisms in an equally posi- 

 tive manner and with fullest possible details. He concludes that with one 

 exception all of Groneman's thirty-two objections rested upon observa- 

 tions made carefully by Tromholt himself, and that, conscious of the 

 favorable location whence he observed, of his many years' experience, 

 and of the special care given to the examination of the whole heavens, 

 he must still remain convinced that in many cases the location of the 

 auroral light is very near to the earth's surface, frequently underneath 

 the clouds and sometimes at the surface. (Z. 0. G. M., xvn. pp. 342-351.) 



S. Fritz has published in Danish a memoir entitled Recent Investiga- 

 tion on Winch in the Atmosphere, &c, in which he gives the results of 

 years of personal experience in reference to the auroral phenomena at 

 Ivigtut, Greenland, 61° 20' north latitude. Within the belt of gieatest 

 frequency the aurora is an almost daily phenomenon that fails only 

 when fog or clouds obscure it. North of 80° north latitude it is rarely 

 seen. Between this and the zone of maximum frequency, it is seen to 

 the south of the observer's zenith, and usually as a freely moving wavy 

 baud of great horizontal extent. This, "the Arctic form," consists of a 

 series of bars of light arrayed side by side perpendicular to the general 

 axis of the band, while the whole band is always in a wavy, often in a 



