442 
NATURE 

[ Oct. 5, 1871 



In all these cases of auroral displays the inductive 
effects upon the telegraph wires are very strongly marked ; 
currents of varying intensity and direction flowing un- 
ceasingly through these metallic circuits. 
The result of observations made in Shetland during the 
months of September, October, November, and December 
last year, tend to show that these auroral disturbances 
attained their maximum effect upon the wires between 
8 30" and g" 30™ A M., and between 8" 30™and 10" 30™ P.M. ; 
and such is the unstableness of these induced auroral cur- 
rents, that frequently in five minutes the electromotive force 
will vary from very much less than that of a Daniell cell 
to a current of such intensity that a brilliant stream of 
light will flash across the points of the lightning conductors 
with sharp detonating reports, the electromotive force of 
which would be scarcely equalled by 500 Daniell cells. 
In January last very curious electrical phenomena were 
observed at Lerwick through the day-time, in connection 
with the N.E. gales so prevalent at that period of the year. 
In Shetland these galesare almost withoutexceptionaccom- 
panied with very severe hail-storms. The day begins bright 
and fine, a clear sky, the barometer rapidly rising ; low on 
the horizon may be observed denseand angry-looking clouds. 
One by one these clouds travel fast towards the zenith, 
when all at once a fearful gust of wind, accompanied with 
the most violent hail-storm, will apparently break out of 
the cloud, and continue for about fifteen minutes. The 
wind then subsides, and the day appears as fine as before. 
In half an hour’s time a second cloud will have appeared, 
and there will be a repetition of the temporary tornado 
and hail-storm. The remarkable circumstance attending 
these successive storm clouds is that they appear to be a 
purely electrical phenomenon. The moment that the icy 
discharge takes place from the cloud with its accompany- 
ing “crack” of wind, an induced electrical current appears 
upon the wire, so strong that it attracts firmly down the 
armatures of the telegraph Morse apparatus. The 
moment, however, that the hail ceases, the current passes 
off, but with this result, that each successive cloud storm 
appears to induce a current flowing inan opposite direc- 
tion from the last, that is to say, the currents appear to 
be (using conventional language) positive and negative in 
their effects. 
That these storms are “ electrically excited” there is no 
disputing, and that they occur during the prevalence of 
the chief auroral displays is also a matter of observation, 
but so far their connection with aurora has not been 
sufficiently determined to permit any opinion to be ex- 
pressed. 
The recent successful completion of the telegraph 
circuit to Shetland, and the extensions immediately to be 
carried out one hundred miles farther north, will afford 
much greater facilities for auroral observation than has 
hitherto existed. It is also proposed to institute a careful 
spectroscopical examination of the coloured scintillations ; 
and now that the Meteorological Society are about to 
establish an observation station in Shetland, there is every 
prospect of some valuable data being collected on this 
interesting subject, which may hereafter cuide our meteoro- 
logical students in arriving at some satisfactory conclusion 
regarding the laws of electrical storms and auroral induc- 
tion. At present we are only able to record a few care- 
fully observed facts, 

THE LIGHT OF FUPITER’S SATELLITES 
Ueber die Helligheitsverhaltnisse der Fupiterstrabanten,von 
Dr. R. Engelmann, Observator der Sternwarte zu Leip- 
zig. (Leipzig; London: Williams and Norgate. 1871.) 
‘ey all the satellite systems which so essentially enrich 
the retinue of the sun, none, when we have left our own 
moon behind us, promises such a reward for investigation as 
that of the planet Jupiter. The remoter ones may be, and 
probably are, intrinsically of a more remarkable charac- 
ter, but they are, and ever will remain to a great extent, 
beyond our reach; while the attendants of the largest 
among the planets are numerous enough to interest by in- 
dividual peculiarities, which their comparative proximity 
enables us to study with advantage. Yet it is readily 
observable that though ordinary telescopes of good quality 
would have done much towards elucidating their pheno- 
mena, very little progress has been made in the inquiry, 
especially in this country ; and the work now before us is 
the first attempt to collect and to make serviceable the 
scattered observations which exist, of which we are sorry 
to remark how few are due to the astronomers of Eng- 
land. 
The especial object of the eminent observer at Leip- 
zig has been not the theory of the motions of these 
satellites, but simply their physical aspect in regard to the 
variable light which they have long been known to re- 
flect, and to this investigation the author, notwithstanding 
constant engagement in important zone observatio 1s, has 
contributed far more than all who have preceded him. 
The instrument which he employed was the astrophoto- 
meter of Zdllner. In this ingenious contrivance, the light 
of the object to be examined is referred to that of one or 
more known comparison stars, by means of an artificial 
star produced by a petroleum flame, adjustable for bright- 
ness and colour by a Nicol prism, and a “ colorimeter,” 
or revolving wheel of tinted rock-crystal. But in order to 
eliminate the effect of unequal areas, so as to ascertain, 
not merely the absolute amount of light reflected, but the 
“albedo,” or reflecting power of each surface, it is, of 
course, necessary {o obtain reliable measures of these 
minute specks of light; and in order to decide the 
interesting question whether or not their rotation 
and revolution are, as with our own satellite, syn- 
chronous, their anomalies, or orbital positions relative 
to their primary, have to be taken into account, All 
this has been done with most praiseworthy care; the 
whole is discussed and reduced with scrupulous and 
exemplary attention to every possible source of accidental 
error ; and the result is given to the eye in several elabo- 
rate diagrams. We shall merely specify some of the con- 
clusions, which will be found of considerable interest to 
astronomers. The absolute brightness was found by the 
author, as it has been by all previous observers, very 
variable ; and from the irregularity and occasional rapidity 
of its changes, it becomes impossible to decide, in the case 
of the three interior satellites, whether the periods of 
rotation and revolution are identical. This, however, 
appears to be decidedly the fact with the outermost, 
Herschel I. had extended the inference to all of them; 
but such a result could not now be accepted ; and it seems 
probable that the spots which must occasion these varia- 
tions, and which have been repeatedly noticed when the 

