)) J J) 



332 Mr. J. Rand Capron on the Auroral 



as given in the first formula, of which the last two are the 

 criteria to show how far such a mean differs from the use of 

 the rule directly on each side of the shadow-line. The result- 

 ing figures are found to be: — 



Formula No. 1 h=lS2'6 miles. 



2 A=128'2 „ 



3 h= 136-9 „ 

 And the mean of Nos. 2 and 3 gives us 



h = 132 55 miles; 



in curiously close accord with that obtained by actual projec- 

 tion (fig. 2), viz. 133 miles. 



The course the beam pursued would be, at 185 miles 

 from the shadow-line (fig. 1), passing from a little S. of 

 Dresden to Frankfort, then between Bruxelles and Paris, 

 but nearer the latter place and a little N. of it, across Evreux 

 to Quimper ; while at 244 miles (fig. 2) we should find it 

 begin at Prague, then to Worms, across Fontainebleau a little 

 S. of Paris, to Nantes. In either case the line would be 

 slightly curved, from its getting into regions where the mag- 

 netic variation becomes gradually greater towards the west. 

 As to the speed of motion, a path of between 800 and 900 

 miles long seems to have been traversed in some 80 or 90 

 seconds, giving a rate of 10 miles per second. 



Prof. Herschel concludes by remarking that he has seen 

 abortive bright streamers move along stationary milk-white 

 auroral bands; and this " shuttle phenomenon " was, he thinks, 

 a streamer-base, or "tendency to shoot up rays," travelling as 

 a nucleus or concentration of light along an arc or bow other- 

 wise invisible, as streamer-bases, though more faintly visible, 

 frequently do along luminous sharp-edged phosphorescent- 

 looking arches. It will be noticed that this last remark of 

 Prof. Herschel's well accords with Mr. Joseph Clark's and 

 my own observations — that the beam seemed to be following 

 a definite and, as it were, " bespoken " track. 



It may here, too, be remarked that observers who did not 

 notice the "beam" do mention an arch or arches at about 

 5 h 30 m , which, except for the former's flying evanescent cha- 

 racter, must have much resembled it. 



Two such observations by Mr. Clark and Mr. Clapham 

 (obs. 22 and 23) are comprised in the particulars scheduled. 

 Mr. Clark speaks of a " green" arch, a colour which seems to 

 have been particularly dominant in the 17th of November 

 aurora. Mr. Clapham describes a stationary arc formed of 

 two conical-shaped lights, with apices meeting about the 

 zenith. Dr. Roxburgh, in his letter to Prof. Smyth before 



