Ferruaky 14, 1907] 
NATURE 
367 
descriptions. The author has prepared for this con- 
tingency by providing references under each genus 
to the “ Flora Australiensis ’’? and the ‘‘ Flora of New 
South Wales,”? and has arranged his system and 
nomenclature according to the last named. Ferns 
and fern allies are included, but of monocotyledons 
the families of rushes, sedges, and grasses are left 
out. 
LETTERS TO THE EDITOK. 
[The Editor does not hold himself responsible for opinions 
expressed by his correspondents. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice is taken of anonymous communications.| 
Magnetic Storm and Aurora on February g-1o. 
A MAGNETIC storm was recorded at the Kew Observatory 
(National Physical Laboratory) on the afternoon of 
February g and early morning of February 10 larger than 
any that has occurred since October 31, 1903. The curves 
were slightly disturbed during the whole of February 9g, 
but the storm may be regarded as commencing with a rapid 
movement of a few minutes of are in the declination 
needle at 2.15 p.m., with a synchronous sudden rise of 
45 7 (1 y=o00001 C.G.S.) in the horizontal force. The 
storm lasted an unusually short time, being practically over 
by 3 a.m. on February 10, but several large rapid move- 
ments were recorded. The largest declination movement 
occurred between 8.19 p.m. and 8.45 p.m. on February 9. 
During these twenty-six minutes the needle moyed 57’ 
to the west and then 73/ to the east, the extreme westerly 
pesition being reached at 8.34 p.m. The most easterly 
position during the storm was reached at about 10.55 p-m., 
when the trace was off the sheet for a few minutes. The 
range during the storm actually shown on the sheet was 
1° 38’. Between 1.13 a.m. and 1.45 a.m. on February 10 
the needle moved steadily, without sensible oscillation, to 
the west, this movement reaching 1°. The rate of move- 
ment was practically uniform from 1.13 a.m. to 1.33 a.m., 
when it accelerated so suddenly that the curve resembles two 
straight lines inclined at a finite angle. 
In the case of the horizontal force, the force fell more 
than 355 y between 8.25 p.m. and 8.33 p.m. on February 9, 
when it went off the sheet for a few minutes. Between 
8.40 p-m. and 8.49 p.m. it increased fully 240 y. The total 
range during the storm exceeded 480 7. 
The vertical force, though less disturbed than the other 
elements, showed a range of 325 y, the highest and lowest 
values being attained at 6.25 p.m. on February 9 and 
1.48 a.m. on February 10 respectively. The most rapid 
change took place between 8.25 p.m. and 8.42 p.m. on 
February 9, The storm was doubtless associated with the 
aurora, which seems to have been widely observed on the 
evening of February 9. CHARLES CHREE. 
Aw unusually beautiful display of aurora borealis was 
seen here (51° 56’ N. lat., 2° 35’ W. long.) between 
6.30 p.m. and 11 p.m. on Saturday evening, February 9- 
At about 6.30 p.m. I became aware that the north-western 
sky, instead of darkening after sunset, was becoming 
lighter, and the quivering upward rays showed that it was 
the northern lights. The aurora was at its best between 
8 p.m. and g.30 p.m., stretching half across the northern 
heavens from Cetus to Leo, from the horizon upwards 
towards the zenith, some of the curved flashes reaching to 
Jupiter. 
This aurora was characterised by the brilliant soft white- 
ness of its light, occasionally tinged with pale green, 
which filled the north-western and northern sky from the 
horizon to a considerable elevation, from which at times 
long rays shot up; but more generally the lights appeared 
as curved, wavy bands rushing up to the zenith, and hang- 
ing there for a few seconds as white, cloudy patches in 
the clear sky among the brighter stars. Between 8.45 p-m. 
and 9.15 p.m. the colour about Ursa Major and Leo was 
a dull, faint red. The aurora was not watched after 
NO. 1946, VOL. 75] 
11 o'clock, but by that time it had greatly diminished in 
brilliancy, and the sky was becoming cloudy. 
I may add that for some weeks L have been noting the 
sun-spots, of which lately there have been a considerab‘e 
number, and on the morning of February 9 one near the 
middle of the sun’s disc was so large that 1 afterwarcs 
saw it with the naked eye through smoked glass. 
B.A. 
Dadnor, near Ross, Herefordshire, February 11. 
The Flight of an Elongated Shot. 
Woutp any reader of Nature kindly enlighten me on 
the following points in the theory of projectiles? 
(1) Whether one is right in supposing that a bullet or 
shot of the modern pointed cylindrical form, when fired at 
any angle of elevation in vacuo, would preserve the 
original direction of its axis of rotation, so that at the 
end of its flight its long axis would be considerably in- 
clined to its line of flight. 
(2) Whether a similar shot fired through the air would 
be acted upon by a couple tending to produce rotation 
about an axis perpendicular to the plane of the trajectory, 
the magnitude and direction of this couple depending upon 
the form of the projectile and the position of its centre 
of gravity, a zero value being possible; and whether the 
effect of this couple would be to produce rotation about 
an axis in the plane of the trajectory and perpendicular 
to the long axis of the shot, so that the point of the 
projectile would be deflected downwards and to the right 
or left. 
(3) Whether, if the above suppositions are correct, any 
successful attempts have been made to keep the long axis 
of the shot tangential to its trajectory during the whole 
course of its flight, by giving it a particular form, and 
varying the density of its parts in a particular way. 
P. D. STRACHAN. 
Philippolis, Orange River Colony. 
Tne answer to proposition (1) is best given for the most 
general case. A body projected in any manner in a field 
of gravity in vacuo will move so that the centre of gravity 
(C.G.) describes a parabola, while the body moves about 
the C.G. so that to an observer seated at the C.G. the 
body has the motion described by Poinsot, in which the 
momental ellipsoid rolls on a fixed plane. The normal to 
this plane is the axis of resultant angular momentum, and 
this axis preserves 2 direction fixed in space, while the 
body moves about it. When this axis coincides with a 
principal axis, the body appears to be spinning steadily 
about the axis, but a closer observation reveals always 
a precessional and nutational motion. 
The question in the limited form of proposition (1) pre- 
supposes a body of perfect uniaxial symmetry spun 
accurately about its axis; but such a condition cannot be 
realised in practice any more. than it is possible to balance 
a pin on its point, asd so it is better to replace this ideal 
state of proposition (1) by the penultimate state, in which 
the spinning body, like a sleeping top upright, has steadi- 
ness almost nerfect. 
With this limitation the axis of an elongated shot would 
move parallel to itself. on the whole, if fired in a vacuum 
as stated in proposition (1). But if fired in air, as in 
proposition (2), a couple arises as soon as the axis is 
oblique to the direction of motion, tending to place the 
axis of an elongated shot broadside to its motion and at 
right angles to the tangent of the trajectory, and this couple 
acting on the rotating shot will cause the axis to precess 
about the tangent. Even in the absence of air resist- 
ance and gravity, the resulting motion is of great com- 
plexity where the body is influenced by the stirring up 
of the surrounding medium, and the special case of a 
figure of revolution, discussed by Kirchhoff and Clebsch, 
is more complicated than the gyroscopic motion of a top 
spinning in a smooth cup. 
The problem defies analysis when gravity and air resist- 
ance are taken into account: all we can say is that the 
frictional drag damps the nutation, and causes the axis of 
the shot to follow the tangent of the trajectory very 
closely, the point of the shot being seen to be slightly 
above the tangent and to the risht, with a right-handed 
spin. The conditions of proposition (3) are secured then 
