70 
NATURE 
[WVov. 17, 1881 
7.20 on the 13th; and the third from 11, §0 a.m on the 13th to 7 
to 8 a.m, on the 14th of August. 
I have prepared a large sheet, on which these curves have been 
copied as accurately as possible for the first of these storms on 
the 11th, For this storm I have also the curves from Toronto 
and from Zi-ka-wei, The first storm began on August 11 at the 
same instant at all the stations, There is a decided similarity, 
especially in the horizontal force curves, throughout the first part 
of this storm, and certain points in it stand out prominently, At 
Kew, the beginning of the storm is not actually recorded, because 
the sheets of prepared paper on the time cylinders were changed 
pee at 10,20 a.m, when the storm was beginning, The de- 
ections are alike at Lisbon, Kew, Vienna, St. Petersburg, and 
after the very first sudden deflection, at Toronto also. The 
greatest effect is produced at St, Petersburg; the similarity 
between the large disturbances at Vienna and at Toronto in 
Canada, places differing about 64 hours in time, is remarkable. 
About 11.45, 1 p.m., and 2,40 p,m,, there are very remarkable 
points of agreement. 
From about 4.30 p.m, to 8 p.m. Greenwich time, #e., from 
about If am, to 2,30 p.m. Toronto time, the deflections are 
opposed at Toronto, and at Vienna or Kew, 
his would rather point to solar action as the cause of disturb- 
ance, In this case the Kew curve is not so much deflected as 
the Vienna curve, because the horizontal needle at Kew is not 
nearly so sensitive as at Vienna, and the relative strengths of the 
actual disturbing forces at the two places can only be obtained by 
comparison of the scale-values at the two places, 
I will draw your attention to one other point on this day, At 
9 p.m, the disturbances are all in the same direction, but about 
11 p.m, whilst St. Petersburg agrees in direction with the others 
in a very violent phase of the storm, at Toronto the direction of 
the deflections is reversed, and this reversal of curves continues 
until about the end of the first of the three storms. 
The second storm, beginning about 11,30 a.m, on the rath, and 
lasting until the next morning, was the most remarkable of the 
three, 1t not only bafiles the telegraph clerks, who wish to keep 
out earth currents from their lines, but it even goes beyond the 
powers of the magnetic observatories which are specially designed 
to watch over them, Thus, at Toronto, the line goes off the 
edge of the paper on which the photographic record is taken, At 
Melbourne the motion is so rapid, and also at Vienna, that the 
plate is not sensitive enough to receive the impressions; the 
motion is too quick even for photography, At the time of 
greatest disturbance, about 12,20 midday, it is very remarkable 
that at Lisbon, and at Zi-ka-wei, near Shanghai in China, two 
places nearly in the same latitude, but nearly nine hours apart in 
time, the vertical force is increased in precisely the same way and 
to the same amount at the same instant. 
At Zi-ka-wei in China, the sudden change in the horizontal 
force on the needle amounted to about y}oth part of the total 
horizontal force, and at St. Petersburg the change in the hori- 
zontal force amounted to y4th part of the horizontal force, and 
the total force was changed by about glyth part of its full value, 
Hence, any cause for these magnetic changes, in order to be a 
true and sufficient one, must be capable of producing such intense 
magnetic changes as these all over the surface of the earth, 
These magnetic changes are so large as to be quite comparable, as 
we see, with the earth’s total force, so that any cause which is 
shown to be incompetent from the nature of things to produce the 
one can hardly be held to account for the other. 
Since, as I have shown, the large disturbances and the small 
disturbances do not follow totally different laws, but agree 
equally well all over the earth, in so far as they agree we must 
attribute them to the same cause. 
During this August storm, as also during the remarkable 
storm of January 31 last, great difficulties were experienced in 
working the telegraph lines, and Mr, Preece has been kind 
enough to send me particulars of these storms, 
I am also greatly indebted to the Astronomer-Royal for 
sending me traces of the earth-current photographic records 
taken at Greenwich Observatory during the August storm on 
two separate wires, one running from the north-east, and the 
other from the south-east, to Greenwich, The two tracings are 
bent opposite w 7 at the same time, so that when a current was 
running on one line towards Greenwich, on the other it was 
running away from it, and comparing these curves with the earth- 
current records from Derby and Haverfordwest and other places, 
it appears that the general direction of currents during this storm 
was from south-west to north-east, or from south-south-west to 
, north-north-east, with varying intensity, the agreement 
| very close between the disturbances of the declination neotle SN 
| the Blackheath and Greenwich photographic record, From Mr, 
Preece’s record also earth-currents were violent from 10,30 a.m, 
on the 11th (i¢ they were noted within ten minutes of the begin- 
ning of the magnetic storm) to about 2.30 p.m., and again from 
9 to midnight, 
They were very violent on August 12, beginning at 11,30 a.m.,, 
the beginning of the second storm, and quieting down about 
4.30 p.m, then beginning again at 7,30 and lasting until — 
9.30 p.m, 
| Again on the 13th they are strong for 14 hours, from about 
5 in the morning, ¢,¢, just about the end of the second magnetic 
storm, 
The general direction of the earth-currents as observed at 
Derby or Haverfordwest, as well as at Greenwich, was from 
north-east to south-west. ; 
Again on erp 31 last another violent magnetic storm oc- 
curred, in which the currents were even more violent than in the 
August storm, 
Intimately connected with magnetic disturbances and earth 
currents is the phenomenon of the aurora or polar light, which 
is an electric discharge in the upper regions of the atmosphere, 
During the August and gm storms the aurora was well 
seen in England; it was also seen at St. Petersburg, and as far 
east as Siberia, It does not appear to have been seen, although 
it was looked for, at Zi-ka-wei in China by M, Dechevrens, n 
director of the observatory, although the magnetic storm was 
so violent there that the horizontal force was suddenly changed 
by yfoth part of its total amount. 
We may arrive at some idea of the character of the aurora by 
studying electric discharges in vacuum tubes, and Dr, De La Rue 
has already brought this subject before you in his Friday evening 
lecture. 
We may gradually pass from electric discharges in air of 
ordinary density, in which we get the well-known electric spark 
between two surfaces, to air of less density but better conducting 
power, and then to air of less density still, but of such high 
resistance that no electricity will pass. Dr. De La Rue has 
shown that with 11,000 cells of his battery the striking distance 
between two points is about six-tenths of an inch in air of 
ordinary density of about 760 mm, pressure. 
When the pressure in a hydrogen tube is reduced to 21.7 mm. 
8,937 cells will cause a discharge to take place through thirty 
inches. 
When the pressure is reduced to “642 (about six-tenths of a 
mm.), 430 cells will cause a discharge through the tube. 
When the pressure is still further reduced to ‘0065, it requires 
8,937 cells to cause the discharge, so that the spark passes more 
readily at a pressure of “642mm. than it does at a higher or 
a lower pressure, This is also the case with air, 
The lower regions of the earth’s atmosphere offer great 
resistance to the passage of electricity, but as we ascend the 
pressure diminishes and the electric resistance diminishes, until 
at last, at a height of between thirty and forty miles, a level 
is reached where the air offers least resistance to the passage 
of electricity, where the pressure is about *397 of a mm., and 
above that level the electrical resistance again increases, so that 
at a height of about eighty miles the battery of 11,000 cells 
would not cause a spark to pass. 
If we take a tube which has not been very highly exhausted 
we see that the light from the positive pole extends nearly 
through the tube, and the dark space around the negative pole 
is small, As the exhaustion proceeds and the pressure of the 
air is diminished, the electric spark passes through greater and 
greater lengths and changes its character, until we get to the 
pressure corresponding to the least resistance, Beyond that the 
resistance increases, the dark space around the negative pole ex- 
pands and the molecules fly about more freely ; those on the nega- 
tive plate being charged with electricity, and being repelled from 
it proceed for a long distance in straight lines, and possess the 
power of causing bodies on which they strike to glow. In Mr, 
Crookes’s tubes we get very beautiful effects from this glowing of 
the glass tube itself, or from the glowing of substances in the 
path of the stream, We may regard this as a stream of mole- 
cules of gas charged with electricity, and we see the difference 
between this stream and the electric current ina vacuum tube at 
lower exhaustion by the action of the magnet upon it. In one 
case the current going through the molecules from pole to pole 
in the tube is bent out of its course by the magnet, and symmetri- 
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