STOKES. — AURORAL OBSERVATIONS. 19 



A copy of Capt. Hoffmeyer's weather charts is supplied, and it 

 will be evident from an inspection of these charts of what im- 

 portance accurate observations in high latitudes will be for the 

 subsequent study of the weather of Europe during the period 

 embraced by the stay of the Expedition in the Arctic seas. 



2. Note on Auroral Observations. By Prof. Stokes, 



Sec. E.S. 



The frequency of the Aurora in Arctic regions affords peculiar 

 facilities for the study of the general features of the pheno- 

 menon, as in case the observer thinks he has perceived any 

 law, he will probably soon and repeatedly have opportunities of 

 confronting it with observation. The following points are worthy 

 of attention. 



Streamers. — It is well known that, at least as a rule, the 

 streamers are parallel to the dipping-needle, as is inferred from the 

 observation that they form arcs of great circles passing through 

 the magnetic zenith. It has been stated, however, that they have 

 sometimes been seen curved. Should anything of this kind be 

 noticed, the observer ought to note the circumstances most care- 

 fully. He should notice particularly whether it is one and the 

 same streamer that is curved, or whether the curvature is apparent 

 only, and arises from the circumstance that a number of short, 

 straight streamers start from bases so arranged that the luminosity 

 as a whole presents the form of a curved band. 



Have the streamers any lateral motion, and if so, is it from 

 right to left or left to right, or sometimes one and sometimes the 

 other, according to the quarter of the heavens in which the 

 streamer is seen, or other circumstances ? Again, if there be 

 lateral motion, is it that the individual streamers move sideways, 

 or that fresh streamers arise to one side of the former, or partly 

 the one and partly the other ? Do streamers, or does some por- 

 tion of a system of streamers, appear to have any uniform relation 

 to clouds, as if they sprang from them ? Can stars be seen imme- 

 diately under the base of streamers ? Do streamers appear to have 

 any definite relation to mountains ? Are they ever seen between 

 the observer and a mountain, so as to appear to be projected 

 on it ? This or any other indication of a low origin ought to be 

 most carefully described. 



When streamers form a corona, the character of it should be 

 described. 



Auroral arches. — Are arches always perpendicular to the 

 magnetic meridian ? If incomplete, do they grow laterally, 

 and if so, in what manner, and towards which side ? Do they 

 always move from north (magnetic) to south, and if so, is it 

 by a southerly motion of the individual streamers, or by new 

 streamers springing up to the south of the old ones ? What (by 

 estimation, or by reference to known stars) may be the breadth of 

 the arch in different positions in its progress ? Do arches appear 

 to be nothing but congeries of streamers, or to have an indepen- 



