536 
NATURE 
[JuLy 15, 1915 


view that as long as plants grow so long will cellulose 
be formed, and with this as a basis nitro-cotton can 
certainly be prepared. I do not wish to convey the 
idea that the reason why cotton waste is chosen tor 
making nitro-cotton is its chemical idiosyncrasies, but 
rather that it is the most abundant and fairly uniform 
stuff which is available for practical use. Mr. Balls 
is quite right in thinking that the physical structure 
of cotton fibre has much to do with the applicability 
of cotton for making propulsive explosives, but this 
cuts both ways, because even cotton is troublesome 
in that those fibres tend to retain acid, which has to 
be removed by regulated boiling and washing. Hence 
the suggestion that an artificial ‘cotton’? might be 
used is well worth considering—dissolved and squirted 
cellulose being necessarily fairly uniform—and I agree 
with Mr. Balls that it would not only be possible but 
also tolerably easy to turn out perfectly uniform gun- 
cotton made this way. 
Nobody has denied from the beginning of the dis- 
cussion which has taken place in the Press on the sub- 
ject of cotton for explosives, that nitro-cotton can be 
prepared from any source of nature which can provide 
us with cellulose, but the issue is really rather different. 
It is impracticable for a factory accustomed to using a 
particular raw material so to alter its operations as 
to use another raw material without great delay, 
expense, and in this case much danger. The other 
difficulty has been mentioned by Mr. Balls, and is 
that of the artillerist who, given even a better nitro- 
cotton than that to which he is accustomed, would 
have to learn his art all over again, and meanwhile it 
is of more advantage to his country that he should be 
sighting his gun in the manner to which he is accus- 
tomed. To use a homely expression, it is génerally a 
mistale to swap horses in crossing a stream. 
THe WRITER OF THE ARTICLE. 
The Great Aurora of June 16, 1915. 
THERE have been comparatively few auroras here of 
late years. In that time we have passed through the 
aurora minimum. Lately, however, there have been 
distinct indications of an awakened activity in the 
frequent appearance of auroral glows and arches. But 
these were generally feeble, and at best showed very 
little in the way of streamers or other signs of activity. 
The night of June 16 was very clear throughout, and 
gave us one of the finest displays of the aurora that 
I have seen. Before the moon set there was a very 
strong, low-lying arch in the north. This was not 
active, but its intensity seemed to suggest the possi- 
bilities of a great display. 
At 15h. 30m. G.M.T. the arch was very low and 
flat, and extended a great distance towards the east 
and west. There were no streamers. 
At 16h. 10m. there were no streamers, but there was 
considerable action underneath the arch in the way of 
brightening masses. At 16h. 50m. the arch was 
double in its eastern part, the upper portion extending 
to the lower part of Cassiopeia. 
By 17h. tom. the aurora was very brilliant and 
active, the arch having risen and spread all over the 
north, nearly as high as the pole. The lower part of 
the arch was broken with bright moving forms. A 
few minutes later the arch had risen above the pole, 
while below it there was very little auroral light except 
a few streamers. By 17h. 30m. the aurora had 
quietened down, and the great spasm, seemingly 
caused by the rising of the arch, had subsided, with 
the exception of some great bright masses in the 

north-west, where the remnants of the arch had 
drifted. 
By 18h. 13m. a low, strong arch, slightly active, 
NO. 2385, VOL. 95| 


had again formed in the north. The new arch soon 
rose and also became double. 
By tgh. 43m. the arch was breaking up, and very 
long streamers were ascending everywhere. by 
igh. 52m. quick waves of light were ascending towards 
the zenith, succeeding each other with remarkable 
rapidity. hese waves continued without intermission 
until dawn blotted them out about 21h. 5m. 
Atter about 19h. 40m. the display rapidly increased 
in magnitude, and attained its maximum splendour at 
about 20h. 1om. or 15m., when it became too be- 
wildering to describe. The whole heavens seemed 
alive with flickering and dancing light. The rapid 
waves of light and the streamers were ascending in all 
directions, even from the south, to a _ focus 
appeared to be on or near the meridian and about 20° 
south of the zenith. (Its declination was roughly 
+23°.) This focal spot was occupied by some of the 
irregular luminous masses which were momentarily 
brightly illuminated by these waves. The streamers 
were no longer slender rays; they were now broad 
sheets of light along which the ascending waves raced 
with intense rapidity, giving them the appearance of 
flames rushing up frém the horizon to and beyond 
the zenith. These quick light waves momentarily and 
brightly illuminated every object over which they 
passed. The sky was full of wisps and curved streaks 
of luminous matter over which the light waves took 
a sensible moment to pass. This produced a remark- 
able effect. As the light ran from end to end of them 
the rapid brightening seemed to give life and action 
to these streaks and produced in them a writhing and 
darting motion which they did not possess, for their 
real motion and change of form was quite sluggish. 
As late as 21h. 25m., when the sky was bright with 
dawn, some of the great bright masses were still 
visible in the north-west. 
There was but little colour in the display at any 
time, though some of the streamers and masses 
assumed a slight pinkish tinge at about 19 hours. 
Efforts were made several times to form curtains at 
the bases of masses of streamers, but no regular cur- 
tains were actually formed. 
Throughout the night I was photographing with the 
Bruce telescope. As frequently as possible notes were 
made of the progress of the aurora. At 20h. om. the 
sky was so brilliantly lighted that I was forced to 
close the exposure. The resulting negatives were 
badly fogged with the auroral light, which seemed to 
be more actinic than moonlight. 
The present aurora was much like that of 1903, 
October 30 (see Astrophysical Journal, vol. Xxxi., 
p- 212), in its phenomena and its effects on the tele- 
graph systems, but exceeded it in some respects. 
From the newspaper accounts we learn that the 
aurora was visible over the greater part of the United 
States and Canada. It was strong even in California. 
A despatch to the Los Angeles Tribune of June 18, 
from Spokane (Washington), dated June 17, says :— 
“Electrical currents caused by the aurora borealis 
almost stopped telegraph service in northern Idaho, 
Montana, and the Dakotas between midnight and 
1 o'clock this morning. Up to 9 o’clock this morning 
the Western Union Telegraph Company reported in- 
terrupted service, but not to such a great extent as in 
the hour following midnight.’’ 
In the Chicago ‘Tribune of June 18 a despatch from 
New York, dated June 17, says :— 
“Following the spectacular appearance last night of 
the aurora borealis, which lighted the entire north- 
west, came reports to-day of crippled telegraph and 
cable service. The effect of the electricity from the 
northern lights was felt on wires in a zone stretching 
across the continent, and as far south as Pittsburgh. 
that _ 
