434 
NATURE 
— [March 8, 1883 
Influence of a Vacuum on Electricity 
THE theory of Prof. Edlund that a perfect vacuum is a 
perfect conductor of electricity, but that a discharge across such 
a vacuum between two electrodes is prevented by an electro- 
motive force at the surface of the electrode, involves our attri- 
buting to the vacuum the property of screening from electrical 
influence any body which it envelops. If the vacuum be a 
conductor, what we call induction cannot take place through it. 
Not having been able to find any record of an experiment 
which conclusively proved that a vacuum so perfect as to offer 
considerable resistance to the passage of a 
current nevertheless permitted induction to 
take place through it, I have tested the 
matter by means of the apparatus shown in 
the figure, 
AB is a glass tube about 15 cm, long ; 
c is a light hollow platinum ball, 1 cm. in 
| diameter, hung by a fine platinum wire 
from the top of the tube between D and 
‘ | E the two separated halves of a cylindrical 
platinum box, which are insulated from 
each other and held in positiou by platinum 
connections sealed into the sides of the 
| tube, and projecting to the outside at F 
ath and G. 
It is of importance to mention that the 
upper terminal H, from which the sphere 
hung, does not reach more than about 3 
G millimetres above the inner surface of the 
= tube. The two halves of the cylindrical box 
\ are sufficiently near together to prevent the 
ball coming in contact with the sides of the 
SS B glass. 
This tube was exhausted until an induc- 
tion current, would give a 12-millimetre 
spark in air, rather than pass between two 
terminals, K L, sealed in the upper part 
of the tube with their opposed ends about 
half a centimetre apart. 
A wire about 30 cm. long was then hung 
from F, and an electrified body presented to 
the lower end. On the approach of this 
body to the wire the sphere was at once 
attracted towards p, and when a discharge was permitted be- 
tween the electrified object and the wire, the sphere was violently 
attracted, and a minute spark was seen when the wire holding 
it touched the cap of the box p, The sphere was then repelled 
by the similarly charged box, 
It thus appears that the phenomena of electric induction take 
place acro-s a discharze-resisting vacuum, and that the sphere 
hung in it is not screened from electrical influence as it would 
be if surrounced by a c mductor. A. M. WORTHINGTON 
Clifton College, Biistol, February 22 
The Meteoroid of November 17, 1882 
THERE has already been much discussion on this subject, but 
I do not think that such exceptional phenomena lose any of their 
interest by haying happened a few months ago; and so I write 
partly to correct a misapprehension on the part of Mr. Back- 
house and Mr. Groneman as to the bearings of the positions of 
appearance and disappearance of the meteoroid as seen by my- 
self, It seemed to me to appear in the S.E.E. and disappear 
S.W. by S., but these are not the directions of those points 
where the trajectory and the horizon would intersect. By 
mentally continuing the apparent path down to the east and 
west horizons, points would be reached, nearly, but I think not 
quite, 180° apart, the former about 20° N. of E. and the latter 
nearly opposite, so that I scarcely think that it was a great 
circle, but it is very uncertain. Mr. Saxby states that a similar 
cloud was observed to cross the zenith of Brussels by M. Mon- 
tigny. Now there are two accounts—one by M. Zeeman of 
Ziericksee and the other from near Rye (Sussex)—which seem to 
consistently apply to one and the same thing, for the latter place 
is W. by 20° S. from Ziericksee, and from both places the same 
elevation of about 50° was reached. ‘These accounts, if com 
bined with that from Brussels, indicate a height of about 70 
miles ; but then how does such a height agree with some of the 
English observations ? On the supposition of the above height, 
the altitudes of culmination as seen from Woodbridge and 
Windsor would be about 29°, from Bristol 25°, and from York 
10° only, which last angle is directly at variance with the actual 
one. For my part, I will give up the reconciling of such con- 
tradictory evidence to those who have an aptitude for conun- 
drums. The evidence is in favour of this being an auroral 
manifestation, but the spectrum of the cloud does not prove this, 
for as yet it is not known whether the extremely rarefied upper 
atmosphere may not be excited to such incandescence as will 
yield the so-called ‘‘ auroral” spectrum by other means than 
the electric discharge, as, for instance, by the passage cf a cloud 
of meteorites. Mr, Petrie upholds the latter hypothesis, but I 
think that there is a simple but weighty objection to it ; for it is 
difficult to see how acloud of meteoric dust of such closeness 
and defined form as the appearance of this cloud would imply, 
could travel through space for any length of time without 
coalescing into one granular lump, owing to the mutual gravita- 
tion of its particles. Of course this objection will have the less 
weight the smaller we suppose the individual particles to be. This 
argument will scarcely apply to the well-known meteor streams, 
whose individual particles are really so very far apart. If this 
“* flying arch”’ was subject to gravity, it certainly had more than 
sufficient velocity to prevent it being appropriated by our earth 
as a satellite, for the tangential speed necessary to a circular 
orbit of 4100 miles radius round our earth would only be about 
4} miles per second, with a period of 14 hours. All interested 
in this phenomenon will no doubt pay more attention to the 
southern sky during future auroras, in hopes of noting some- 
thing more of a similar nature, and they will also look forward 
to seeing a full account of Prof. Lemstrém’s remarkable experi- 
ments on the nature of the aurora, which he has been conducting 
at Sodankyla with such unlooked-for results. 
Heworth Green, York H. DENNIS TAYLOR 
A Meteor 
LAST evening at 9.35 p.m. a remarkably large and brilliant 
meteor was seen from here, appearing at a point about 10° east 
of 7 Canis Majoris, passing slowly over that star in a south-west 
direction, and vanishing a few degrees above the horizon; time 
about three seconds. Its light had a pale green tint, and in 
brightness and apparent diameter it far exceeded Sirius (which 
was particularly bright all the evening), so much so that my 
companions, though not looking in that direction, were instantly 
attracted by the light, and saw it in its splendour. 
R. W. S. GRIFFITH 
Eyeworth Lodge, Lyndhurst, Hants, March 3 
Aurora 
Last night at about ten o’clock there were two beautiful white 
auroral streamers, like the tails of enormous comets, near the 
Pleiades. They were nearly vertical, and slowly moved, in a 
direction parallel to the horizon, towards Orion, after which they 
gradually vanished. There was little wind, and the night was 
bright starlight, after a cloudy day. There was an auroral glow 
like twilight over the northern horizon. The barometer rose 
during yesterday and last night, and stands high. 
JOsEPH JOHN MurPHY 
Old Forge, Dunmurry, Co. Antrim, February 28 
Hovering of Birds 
WITH regard to Mr. W. Clement Ley’s remarks, I have 
already been permitted to explain in NATURE (vol. xxvii. p. 
366) how I had accidentally misunderstood Mr. Airy’s meaning. 
I do not believe that any bird having a greater specific gravity 
than the air can retain a perfectly fixed position iu a calm with- 
out some wing-motion. Mr. Ley ‘‘believes that there is 
nothing in the etymology of the word ‘hover’ that implies 
movement.” This has induced me to look up a somewhat volu- 
minous‘and recent dictionary, in which I find ‘* Hover, v.2. (W. 
hoviaw, to hang over, to fluctuate, to hover). To flap the wings, 
fluttering or flapping the wings with short irregular flights” ; and 
more to the same effect, all indicating movement. J. RAE 
AMATEURS AND ASTRONOMICAL 
OBSERVATION 
Tp labour done by astronomical an.ateurs has had 
no little influence upon the progress of the science. 
The work achieved by them has indeed often been of the 
