Fan. 25, 1883] 
NATURE 
205 
posture of the bird very well for a few seconds, till he 
became aware of my presence and dashed away. I was 
much struck by those instances in which the obstacle that 
caused the upward slant of the wind was only a sea-wall 
or a railway embankment, and especially by the critical 
case (No 12) where the bird was evidently baffled because 
the wind lay along the embankment, not against it, and 
therefore gave no upward current. 
My list includes four cases (3, 4, 5, 6) of rooks and 
gulls ““up-borne’’ on outspread wings, under conditions 
similar to those present in hovering—cases that could not 
be explained by any theory of wzs vzva, but clearly in- 
volved an external mechanical force, which could only be 
that of the wind, sustaining and uplifting the birds. The 
close relation between the “up-borne” and the “ hover- 
ing” action was evident in case (9) where the gulls, &c., 
were up-borne and sailing, while the hawk was poised 
and motionless. 
These observations, as far as they go, appear to indi- 
cate plainly the law which governs the phenomenon in 
question. I think they strongly confirm the theory 
already advanced, that the bird in hovering is upheld by 
a slant upward current of air. A strong wind pressing 
against a slope of ground is necessarily thrown into a 
slant upward current, ‘‘as slopes a wild brook o’er a 
hidden stone.” There may be a downward eddy if the 
slope is precipitous, as one may often feel at the foot of a 
high wall, but the main stream of the air for some con- 
siderable height above the slope is forced to take an up- 
ward slant, with increased velocity, in order to surmount 
the obstacle in its path. 
Given such a slant upward current, it is easy to see 
A G 
that a bird, with the exquisite muscular sense that every 
act of flight demands and denotes, might so adapt the 
balance of its body and the slope of its wing-surface to 
the wind as to remain motionless in relation to the earth. 
The slope of the wing-surface should divide the angle 
between the horizontal and the direction of the slant 
wind-current in such proportion that, if the air were at 
rest, the bird, under the action of gravity, would float for- 
wards, downwards, on outspread wings, with exactly the 
same velocity as that of the wind (in which it remains 
motionless) and in exactly the opposite direction. The 
mechanical conditions are identical in the two cases, 
whether we consider the air at rest and the bird floating 
through it, or the bird at rest and the wind rushing under 
it. In either case the bird has reached, and maintains, 
its maximum velocity, due to gravity, compatible with the 
resistance of the air, which resistance is the same in both 
cases. 
I have heard it objected that the mechanical conditions 
are not the same in these two cases, because in the one 
case the bird has momentum, in the other not. Need it 
be said that momentum is a purely relative possession, 
just as velocity is? In each case the bird has the same 
velocity, and therefore the same momentum, relative to 
the air. The mechanics of the situation, as between bird 
and air, are not affected by the possession or loss of 
velocity (and with it momentum) relative to the earth. 
Perhaps the feasibility of the thing may be best shown 
by a simple diagram. Let AB represent the slope of the 
bird’s wing (viewed from the right side), dividing the 
angle between the horizontal, ac, and the direction of 
the wind, DA. Draw BD at right angles to AB, and take 
AD to represent the force of the wind. Then DB and 
B A will represent the force of the wind resolved perpen- 
dicular and parallel to the slope of the wing AB. The 
resolved part, BA, meeting only the resistance of the 
bird’s head and shoulders and front edge of the wings, 
tends not strongly to push the bird in the direction, BA, 
that is, backwards and a little upwards. But the resolved 
part, D B, which meets the full area of the outspread wings 
and tail, tends powerfully to push the bird ia the direc- 
tion D B, that is, upwards and a little forwards. Then all 
that is required to keep the bird at rest is that the effect 
of the forward force exerted by DB should balance the 
effect of the backward force exerted by B A (both being 
resolved vertically and horizontally), and that the great 
upward force exerted by DB, together with the small up- 
ward force exerted by BA should exactly neutralise the 
downward force of gravity. 
The only difficulty in the way of the slant-upward- 
current theory lies in the statement of the Duke or Argyll 
(NATURE, vol. x. p. 262) that “a hundred times’’ he has 
seen birds hovering “when by no possibility could any 
upward deflection of the wind have arisen from the con- 
figuration of the ground.” My own observations testify 
so consistently in favour of slant upward currents that I 
feel justified in asking for more precise information con- 
cerning the instances alluded to by the Duke of Argyll, 
before relinquishing the theory which I hold. Wherever 
I have seen a hawk trying to remain in one position over 
a plain or slightly undulating ground, the feat has only 
been accomplished by continued vibration of the wings. 
The problem of the “soaring” of birds introduces 
other conditions, which require separate consideration, 
though I believe it will be found that the two phenomena 
of ‘‘soaring” and “hovering”? depend upon essentially 
similar causes. 
(By the bye, does not the provincial name of one of the 
hawks, the “‘ Windhover”’ record the constantly observed 
dependence of the act of hovering on the wind ?) 
HUBERT AIRY 
THE LATE EDWARD B. TAWNEY 
Y the death of this young naturalist English geology 
has lost one of its most enthusiastic and cultivated 
students. Hardly beyond the threshold of his career, he 
had already gained for himself a notable place among the 
geologists of this country, and his friends augured for him 
a future of distinction and usefulness. But in the fulness 
of his promise and in the midst of his work he has been 
struck down so suddenly that few of his friends knew he 
had been ailing until they were shocked and saddened by 
the news of his death. 
Born in 1841, he was the third child of the Rev. 
Richard Tawney, Vicar of Willoughby, Warwickshire, 
who had gained a distinguished place at Rugby, ana had 
been a Fellow of Magdalen College. On the death of his 
father, young Tawney was placed under the care of his 
guardian, Dr. Bernard of Clifton, and received his early 
education there. During these years he seems to have 
acquired a bent towards natural science mainly through 
the influence of Dr. Bernard and Dr. Fox of Brislington. 
He was eventually enabled to gratify this inclination by 
attending the courses of instruction at the Royal School 
of Mines, Jermyn Street, from 1860-63, where he greatly 
distinguished himself. He gained a Royal Scholarship, 
Duke of Cornwall’s Scholarship, the De la Beche Medal 
for Mining, and the Edward Forbes Medal for Natural 
Science, and took Associate’s diplomas in the Mining and 
Geological divisions. ‘ 
With the training in scientific methods thus obtained, 
he soon betook himself to original research, gaining 
experience by excursions at home and by travel abroad. 
In 1872 he accepted the offer of Assistant Curator of the 
