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Feb. 8, 1883 | 
NATURE 
BE 
area. Owing to the friction which the wind experiences in 
passing over the surface of the earth, however, this upward 
current could exist at its maximum only at a considerable height. 
But it is important to observe that there may frequently be a 
considerable amount of current upwards in the regions where 
birds ‘‘hover,” at least in the neighbourhood of a cyclonic 
vortex. But how much lifting force is necessary to sustain, say 
a gull, in the air? A gull moves its wings in ordinary flight at 
from 160 to 200 (double) strokes per minute, and reckoning 12 
inches as the greatest vertical depth through which the bird can 
raise itself by one (double) stroke, we find that it possesses the 
power of raising itself about 180 vertical feet per minute. This, 
however, is less than one-half of the rate at which we have found 
the currents to rise near the centre of a cyclonic depression. 
From this we may judge it is likely enough that birds ‘* hover,” 
or suspend themselves motionless in the air through the influence 
of upgoing currents, which are masked to our observation by the 
fresh winds which accompany them. 
A kestrel may, however, support itself largely by its peculiar 
quivering play of the wings, and J think it must be difficult to 
determine how much support a bird may contribute by such 
motion, when at a height where it is difficult to observe it. 
I have frequently observed gulls ‘‘ hovering” upon currents 
of air which were heaped up by the wind striking obliquely 
upon a rising coast line, in which case the head is turned at an 
angle to the general direction of the wind, so as to face the 
heaped-up and rising currents. Such passing over irregular 
ground are irregular or gusty, and tax the bird’s utmost muscular 
agility to prevent a sudden lateral turning to leeward. in which 
case the rapid flight with the currents may be compared to the 
fall of a stone ty the ground. The same upward direction to 
the atmospheric currents must be imparted by the cortracting 
sides of a converging valley. But when such local forces derive 
aid from the upward currents peculiar to cyclonic winds, atmo- 
spheric conditions favourable to ‘‘ hovering” must, I think, 
frequently occur. 
On the other hand, I cannot conceive it to be possible for 
birds (and I do not think that the third chapter of the ‘‘ Reign 
of Law,” gives any sufficient explanation) to sustain themselves 
motionless on currents of air which are purely horizontal, for in 
such ease there is nothing to compensate—when the wings are 
slanted at the necessary angle to prevent falling—the backward 
horizontal force, and the creature must inevitably be driven 
backwards, DaviD CUNNINGHAM 
Dundee, February 5 
On August 12, 1881, I observed a hawk maintaining an 
apparently stationary position at a height of about 200 feet above 
the surface of flat ground. He was as a matter of course facing 
the wind, which blew, if I remember rightiy, from the west. 
For the most part his wings remained motionless, but now and 
then he fluttered them for a little while. This was over the 
sensibly level plain which lies between Machrihanish Bay and 
Campbeltown Loch, at the southern end of the Mull of Cantire, 
and, curiously enough, on or close to the Duke of Argyll’s pro- 
perty. The exact spot was about a mile and a half eastwards 
from Machrihanish Bay, and about three-quarters of a mile 
northwards from the southern boundary of the plain. There 
could not be any ‘‘slant upward current,” such as Mr, Airy 
supposes, maintaining him in that position ; at any rate, there 
was no sloping ground near. 
I watched this bird for about ten minutes, and he verified in 
aremarkable manner the views I had held on this subject for 
many years, namely, that, given a steady wind blowing with a 
velocity which lies somewhere between certain possibly calculable 
limits, a hawk can remain fcr a time apparently motionless above 
a point ; he is, in reality, descending a slightly inclined plane, 
and requires to recover vertically lost ground by the occasional 
use of his wings. WILLIAM GALLOWAY 
Cardiff, January 30 
In the letters on the above subject that have appeared in some 
recent numbers of NATURE, the writers lead us to believe that a 
current of air is zecessary to enable a bird to ‘‘ hover ” or retain 
when on wing a motionless position. My observations lead to 
an opposite conclusion, as I have often seen both hawks and 
terns remain steadily poised, when there was not a breath of 
wind. That there was no wird where the birds (¢ervms) were, 
was shown by their heads, when hovering, being turned in dif- 
ferent directions, although at only a short diste nce from each other. 
Generally, if not always on these occasions, I noticed that the 
birds spread out their tails in a more or less depressed position, 
as if to counteract any forward movement likely to be caused by 
the wing-motion. J. RAE 
4, Addison Gardens, February 3 
IN reading the letters published in your last issues of NATURE 
with regard to the hovering of birds, it struck me that a very 
similar thing can be seen sometimes, among inanimate objects 
when an imperfect attempt is made to cause ‘ducks and drakes’ 
with a flat stone. I have commonly noticed that the missile; 
curves sharply upwards and for a moment ‘‘hovers”’ as it were 
in mid-air before dropping. In this case and also in the similar 
one of the motion of the boomerang, the slanting upwards and 
the apparent hovering do not require, and need not be due to, 
upward currents, but merely depend upon the force of a hori- 
zontal current of air meeting the inherent force of the moving 
body. It is not unreasonable to suppose a similar simple solution. 
of bird hovering. C. S. MIDDLEMIss 
Linnzus Street, Hull, February 3 
Is there not an error in the letter of NATURE (p. 312)? The 
writer there suggests, as it seems to me, that a bird could main- 
tain a position of rest, with respect to the earth, by a suitable 
slope of the wings against a horizontal wind. Now, as I 
pointed out in NATURE, vol. xxiii. p. 78, such lifting action on 
the part of the wind can only take place in the interval between 
the time when the bird is first launched from the cliff, and the 
time when it has by friction attained the velocity of the wind. 
That this interval is not a long one, is shown when balloons or 
other objects are launched, 
[[t may be well to notice, that if there were wo friction there 
is no lifting power; so that if we object to the above, that 
“*the bird gives such a very small friction with the wind,” we 
thereby do away to the same extent with the lifting power ; just 
as a frictionless ship in a constant stream would be unmoved 
were it sufficiently tapering. ] 
From the above considerations I have been compelled, since 
writing my last letter, to ascribe the hovering power of birds— 
1. To the “‘exquisite muscular sense” by which they can 
take advantage of all upward currents of air, shifting their 
positions for this purpose. In an elastic fluid as the air, I 
imagine that the stream-lines, even over the sea, are far from 
horizontal. I believe the evidence of balloons over the sea goes 
to show this. 
2. Thereis, to use a common expression, ‘‘ flying avd flying ;” 
just asa man can skate without striking out, so can a bird give 
itself some support by quiet movements of wings and tail. 
I may remark that kestrels keep fluttering their wings at 
short intervals while hovering ; they are never still for long. 
So also terns and gulls, as seen from the fixed point of a cliff, 
are always moving and shifting in a quiet way, which may dis- 
guise both a seeking of upward currents and the quiet sort of 
“flying.” W. LARDEN 
Cheltenham College 
Science and Theology 
CAN you tell me by what right the authorities of Cooper’s 
Hill Engineering College, who are in want of a Professor of 
Physics, make it a condition that he should ‘‘ bea Protestant,” 
and should ‘‘attend morning chapel and Sunday services with 
reasonable regularity, showing in this respect a good example to 
the students?” The institution is one supported by the State, 
and is surely bound to respect the principles which underlie the 
State’s dealings with religious matters. The president (or who- 
ever is responsible for these preposterous conditions) may have 
forgotten this fact ; but I cannot believe that the present Go- 
vernment will allow an appointment to be made until all 
“religious” limitations are cancelled from its conditions. As 
the memorandum stands at present, it appears little short of 
insulting to scientific men, C. 
Intelligence in Animals 
Mr. GRENFELL’S letter in NATURE, vol. xxvii. p. 292, re- 
minded me of a statement in vol. iii. p. 308 of Cook’s last 
voyage, where Capt. King refers to the ordinary sagacity of 
bears, described in a ‘‘thousand stories” which he heard in 
