283 



241. Validity of Estimates. 



[Lewis. 



All estimates of this character are, of course, only provisional, and they 

 can claim no validity, as I have heretofore shown, beyond the accuracy 

 with which they represent the data upon which they are based. That all 

 the sethereal elements which I have considered are important, that they 

 are more far-reaching than those Avhich have been introduced into any 

 like discussion of which I have any knowledge, that their influence has 

 been rightly stated, and that they will contribute, by collation with Thom- 

 son's and other estimates, to a more satisfactory solution of many physical 

 problems than is yet attainable, I fully and unhesitatingly believe. Sun's 

 orbital motion, and questions connected with the retardations which change 

 revolution into simple rotation, are among the considerations which seem 

 likely to modify the values that are given in the five foregoing notes and 

 in Note 35. 



Note on the Aurora of April 16-17, 18S2. By H. Garvill Lewis. 

 {Bead before the American Philosophical Society, April 21, 1882.) 



The aurora of Sunday evening, April 16-17, 1882, was probably one of 

 the most remarkable, both as to beauty and scientific interest, that has 

 been observed in this latitude. It is especially noteworthy on account of 

 the brilliant corona which continued well defined for several hours, and 

 whose apparent motion eastward, through space, could, therefore, be 

 determined. Several other unusual features, such as an auroral curtain, 

 and hyperbolic curves of light, were also displayed. The attendant solar 

 and magnetic phenomena have also been of great importance in determin- 

 ing a theory of the aurora. 



The aurora Avas noticed as soon as twilight had ended as a faint glow 

 along the northern horizon. At 8.30 it was a low arch, probably not over 

 10 degrees high. It gradually rose higher, and left a dark segment behnv 

 it. At 10 P. M. the arch was some 20 degrees high, and was constantly 

 increasing in brilliancy. Bright short white acicular streamers now ap- 

 peared in the north, and sometimes rose as high as 40 degrees. These 

 occasionally assumed a reddish color, and were frequently wafted along 

 the arch towards the west. The aurora now fluctuated greatly in brilliancy, 

 sometimes nearly disappearing, and then flashing out brighter than ever. 



At 11.15 the arch had become brighter and much longer, though still of 

 low altitude. Bright acicular streamers were crowded closely together at 

 the western end of the arch, while in the east a second arch was now 

 formed. The auroral arch now began to rise rapidly. At 11.20 the upper 

 arch was 40 degrees high. Long narrow streamers were rapidly forming 

 over the whole northern sky, and were traversed from base to apex with 



PROC. AMER. PHILOS. SOC. XX. 111. 2.1. PRINTED JUNE 6, 1882. 



