388 
position of the body of the bird to the plane of the horizon is 
observable. The miracle is always performed by the use of the 
appropriate means. ARGYLL 
Cannes, February 12 
I AGREE with ‘‘J. R.” that the term ‘‘hovering” is likely to 
be misunderstood. Iused it because it had been used in the 
earlier correspondence in NaTuRE to which I referred. If 
“¢ J. R.” (or any other of your correspondents on this subject) 
has never seen a hawk hanging in motionless poise above a hill- 
side, I would ask leave to refer him to NATURE, vol. viii. pages 
86 and 324, for a description of the act. 
February 19 HUBERT AIRY 
I HAD once a very unusual opportunity of observing accurately 
the flight of buzzards, from the summit of Acro-Corinthus. As 
this unique natural fortress rises sheer from the plain, on the 
side toward Attica, to the height of eighteen or nineteen hundred 
feet, a group of these birds, hanging at that height above the 
surface, were thus brought in a line with the eye. I could 
detect the minutest movement of wings or tail. Again and again 
there were considerable intervals, of many seconds’ duration, 
during which one bird and another would hang, with pinions 
horizontally outstretched, absolutely motionless, neither descend- 
ing nor drifting, but as if his balance in the air were one of deli- 
cately adjusted equipoise. And when, by a just perceptible 
movement of wing, he stirred again, it seemed rather to be to 
change his position than that he needed any kind or degree of effort 
* to maintainit. The kestrel is an unfortunately chosen bird for Mr. 
Hubert Airy’s observation, because though it hangs for a minute or 
two over the same spot watching its prey, it is always ‘‘ by short 
and rapid motion of its wings” ; from which fanning motion it 
has acquired, I think, its popular name of windhover, and not 
because, as Mr. Airy supposes, it is upborne by the wind. But 
were my Corinthian buzzards upborne by the wind? There was 
mone. ‘The day was one of dead calm. No doubt of necessity 
there was some upward current of air from the sun-warmed 
surface of the ground by which the birds profited; but if at 
all sufficient to sustain them, their actual gravity, when in that 
position and so willing it (by which I mean nothing so absurd 
as that gravitation can be counteracted by the ws vite, but that 
by inflating its lungs, and perhaps suspending its respiration, the 
bird may have the power at will of lessening its comparative 
weight in the air), must be very near to that of the atmosphere 
around and underneath them. It is evident that Mr. Airy could 
only claim my observation as being in favour of his theory if 
there had been a breeze from Attica striking against the face of 
the citadel. There was none perceptible ; and I drew the atten- 
tion of my companions to the curious problem presented by such 
an ease of flight. HEwRY CECIL 
Bregner, Bournemouth, February 13 
P.S.—Will ‘you allow me just to mention that the letter 
reprinted from NATURE by Dr. George J. Romanes in his 
‘* Animal Intelligence,” as mine, is by Mr. Merlin, our present 
Consul in Athens. I sent it, but he wrote it, and the observa- 
tion is wholly his. 
The Auroral Meteoric Phenomena of 
November 17, 1882 
Mr. BACKHOUSE remarks in his letter (NATURE, vol. xxvii- 
p- 315): ‘‘ It would be well to ascertain whether such a motion 
{in a curve) would not agree better with the observations of the 
beam than Dr. Groneman’s hypothesis that it wasa straight line.” 
When a straight line lies within or without the (celestial) 
sphere, on whose surface we wish to trace the perspective pro- 
jection of that line (the eye being placed in the centre of the 
sphere), the perspective of the line will of course always be a 
great circle. When inversely the apparent path of the same 
meteor, seen from ome place of observation is a great circle, the 
true path musi lie ina plane. When the apparent paths, seen at 
the same time from two different places, not situated in the 
direction of the apparent path, are both great circles, the true 
path lies in two different planes, and must be a straight line. 
Now Prof. Oudemans at Utrecht says positively that the 
apparent path of the phenomena of November 17 was a great 
circle, cutting the horizon (and also the equator) in two opposite 
points. Of the English observers I will cite Mr. Saxby (p. 86), 
who describes ‘‘the trajectory as much flatter than that of the | 
NATURE © 
” 
stars.” Moreover the general fact is, as I proved in my paper, 
that this trajectory, having been seen of regular form and con- 
sequently probably of equal curvature in its whole length, 
intersected the great circle of the horizon in two opposite points, 
and therefore must have been a great circle itself. The above- 
mentioned condition being fulfilled, I was under the necessity of 
taking the true path as a right one. I think this peculiarity 
indicates the meteoric nature of the phenomenon and of all the 
auroral arcs (/es arcs proprement dites of my theory) showing as 
great circles of the sphere. In fact a curve cited by Mr. Back- 
house, lying at equal height above a terrestrial parallel, will 
show itself ézt i ome case as a great circle, namely where the 
observer is within its plane. From all other places it will be 
seen as a small circle of the sphere. In this case is the apparent 
boundary of an aurora in the north, the arch of the dark segment 
cutting the horizon in two not opposite points. 
I dare not occupy more space to answer Mr. Backhouse 
further on the influence exercised by cosmic matter on terrestrial 
magnetism, and the consequence of the general direction east to 
west of these currents when passing in the neighbourhood of the 
earth, but I think that this direction east to west must be 
deduced from the observed facts. : 
I am much obliged to Messrs. Petrie and Muirhead for their 
information, As to the remark of the former on the spectrum 
observed by Dr, Rand Capron, I think that the auroral cha- 
racter of some phenomena will be proved the best when it 
shows the auroral lines, whatever may be the origin of its light. 
When its other properties point out its meteoric character, a 
strong argument is found in favour of the cosmic theory of 
auroree. H. J. H. GRONEMAN | 
Groningen, Netherlands, February 14 
The Orbit of the Great Comet of 1882 
1 AM very much obliged to tliose gentlemen who have kindly 
given me the information required in my letter published in 
NATURE, vol. xxvii. p. 314. 
They all agree on the same point, which confirms my opinion 
that in all the good observations the same or very nearly the 
same point of the head was observed during the brightest 
appearance of the comet. 
I remarked especially in the sketches shown to me by 
Mr. A. A. Common, who was the first to see the comet 
in England, on September 16, and who continually made 
careful observations of it, that, although the nucleus was 
seen since October 30 divided into two parts, always one of 
these (which I shall call the main part next to the following 
end of the nucleus) remained the brightest. Mr. Common in 
every drawing marks this part with the word ‘‘ brightest.” At 
the Washington Observatory also this same bright point was 
always observed with the transit instrument, as it is stated by 
Mr. W. C. Winlock in his letter (NATURE, vol. xxvii. p. 129). 
Mr. W. L. Elkin, Cape Observatory, in a communication to 
the Astronomische Nachrichten, No. 2490, speaks about this 
orbit. He used the first observation made at the Cape on 
September 8, the observation fof the] disappearance of the 
comet at the sun’s limb on the 17th of the same month, and a 
normal observation on November 17, to calculate either a 
parabolic orbit or an elliptic one; but none of these gave the 
positions of the comet according to intermediate observations. 
Mr. Elkin believes it is possible to take as the most probable 
value of 1 the value 0°0075, and consequently the comet has a 
a 
very long period, while Mr. Morrison in his calculation of the 
orbit had e = 09998968, and a period of 652°5 years. 
As errors of observations are of course inadmissible, it is now 
the question to study what produces such great differences in 
calculating the orbits. 
Are they due to disturbances during the comet’s passage 
through the solar system, and especially at its passage through 
the sun’s corona? or are they due to the hypothesis specified by 
Mr, Elkin and others that the centre of the nucleus is not the 
point gravitating around the sun? ‘This question cannot be 
decided but by a careful discussion of all the positions of the 
comet during the whole period. 
The observations before perihelion are of course very im- 
portant. Unfortunately at the Cape the astronomers were pre- 
vented making observations between September 8 and Septem- 
ber 17 because of bad weather ; but there are some observations 
made in Melbourne and in other observatories before September 
