April 5, 1883] 
NATURE 
535 
I premise that if we know anything about mechanics it 
is certain that a bird without working his wings cannot, 
either in still air or in a uniform horizontal wind, maintain 
his level indefinitely. For a short time such maintenance 
is possible at the expense of an initial relative velocity, 
but this must soon be exhausted. Whenever therefore a 
bird pursues his course for some time without working 
his wings, we must conclude either (1) that the course is 
not horizontal, (2) that the wind is not horizontal, or (3) 
that the wind is not uniform. It is probable that the 
truth is usually represented by (1) or (2); but the question 
I wish to raise is whether the cause suggested by (3) may 
not sometimes come into operation. 
In NATURE, vol. xxiii. p. 10, Mr. S. E. Peal makes 
very distinct statements as to the soaring of pelicans and 
other large birds in Assam. The course is in large and 
nearly circular sweeps, and at each lap some Io or 20 
feet of elevation is gained. When there ts a wind, the 
birds may in this way “ without once flapping the wings” 
rise from a height of 200 to a height of 8000 feet. 
That birds do not soar when there is no wind is what 
we might suppose, but it is not evident how the existence 
of a wind helps the matter. If the wind were horizontal 
and uniform it certainly could not do so. As it does not 
seem probable that at a moderate distance from the 
ground there could be a sufficient vertical motion of the 
air to maintain the birds, we are led to inquire whether 
anything can be made of the difference of horizontal 
velocities which we know to exist at different levels. 
In a uniform wind the available energy at the 
disposal of the bird depends upon his velocity ve/a- 
tively to the air about him. With only a moderate 
waste this energy can'at any moment be applied to gain 
elevation, the gain of elevation being proportional to the 
loss of relative velocity squared. It will be convenient 
for the moment to ignore the waste referred to, and to 
suppose that the whole energy available remains con- 
stant, so that however the bird may ascend or descend, 
the relative velocity is that due to a fall from a certain 
level to the actual position, the certain level being of 
course that to which the bird might just rise by the 
complete sacrifice of relative velocity. 
For distinctness of conception let us now suppose that 
above and below a certain plane there is a uniform hori- 
zontal wind, but that in ascending through this plane the 
velocity increases, and let us consider how a bird sailing 
somewhat above the plane of separation, and endowed 
with an initial relative velocity, might take advantage of 
the position in which he finds himself. 
The first step is, if necessary, to turn round until the 
relative motion is to leeward, and then to drop gradually 
down through the plane of separation. In falling down 
to the level of the plane there is a gain of relative velo- 
city, but this is of no significance for the present purpose, 
as it is purchased by the loss of elevation ; but in passing 
through the plane there is a really effective gain. In 
entering the lower stratum the actual velocity is indeed 
unaltered, but the velocity relatively to the surrounding 
air is zzcreased. The bird must now wheel round in the 
lower stratum until the direction of motion is to wind- 
ward, and then return to the upper stratum, in entering 
which there is a second increment of relative velocity. 
This process may evidently be repeated indefinitely ; and 
if the successive increments of relative velocity squared 
are large enough to outweigh the inevitable waste which 
is in progress all the while, the bird may maintain his 
level, and even increase his available energy, without 
doing a stroke of work. 
In nature there is of course no such abrupt transition 
as we have just now supposed, but there is usually a con- 
tinuous increase of velocity with height. If this be suf- 
ficient, the bird may still take advantage of it to maintain 
or improve his position without doing work, on the prin- 
ciple that has been explained. For this purpose it is 
1 
only necessary for him to descend while moving to lee- 
ward, and to ascend while moving to windward, the sim- 
plest mode of doing which is to describe circles on a plane 
which inclines downwards to leeward. If in a complete 
lap the advantage thus obtained compensates the waste, 
the mean level will be maintained without expenditure of 
work ; if there be a margin, there will be an outstanding 
gain of level susceptible of indefinite repetition. 
A priori, 1 should not have supposed the variation of 
velocity with height to be adequate for the purpose ; but 
if the facts are correct, some explanation is badly wanted. 
Mr. Peal makes no mention of the circular sweeps being 
inclined to the horizon, a feature which is essential to the 
view suggested. It is just possible, however, that the 
point might escape attention not specially directed to it. 
However the feat may be accomplished, if it be true 
that large birds can maintain and improve their levels 
without doing work, the prospect for human flight be- 
comes less discouraging. Experimenters upon this sub- 
ject would do well to limit their efforts for the present to 
the problem of gliding or sailing through the air. When 
aman can launch himself from an elevation and glide 
long distances before reaching the ground, an important 
step will have been gained, and until this can be done, it 
is very improbable that any attempt to maintain the level 
by expenditure of work can be successful. Large birds 
cannot maintain their levels in still air without a rapid 
horizontal motion, and it is easy to show that the utmost 
muscular work of a man is utterly inadequate with any 
possible wings to allow of his maintenance in a fixed 
position relatively to surrounding air. Witha rapid hori- 
zontal motion, the thing may perhaps be possible, but for 
further information bearing upon this subject, I must 
refer to a paper on the resistance of fluids published in 
the Philosophical Magazine for December, 1876. 
March 22 RAYLEIGH 
PHILIP CHRISTOPH ZELLER 
atom )LOGY has just sustained an irreparable loss 
by the death of Prof. Zeller, which took place at 
Griinhof, near Stettin, on March 27, suddenly, from heart 
disease. Zeller was born on April 9, 1808, at Steinheim, 
in Wiirttemburg. For many years he was attached to 
official educational establishments in Germany, especially 
at Glogau in Silesia, and Meseritz in Posen. While at 
the former place the honorary title of Professor was 
bestowed upon him by the Government on account of his 
eminent scientific researches, and some time afterwards 
he retired from official duties, and settled near Stettin, 
where much of his leisure was devoted to the Ento- 
mological Society that has its headquarters in that town, 
of which he was acting secretary, and of which Dr. C. A. 
Dohrn is president. Zeller’s fame as an entomologist is 
more especially based upon his publications on Lefi- 
doptera, more particularly of Europe, and chiefly on the 
smaller moths. His first recorded paper appeared in 
Oken’s /szs for 1838, and consisted of a critical deter- 
mination of the Lepidoptera in Réaumur’s “ Memoirs,” a 
prize essay, in which the author took first place. From 
that time a continuous stream of valuable papers by him 
appeared, and on the day of his death he was engaged in 
scientific work. It is utterly impossible to give here even 
the titles of his more important works. It is with regret 
that we are obliged to admit that the title of “entomologist” 
does not always enable us to take for granted that the 
entomologist is also a naturalist. Zeller was both, in the 
fullest acceptation of the terms. While his purely de- 
scriptive work is of the highest character, his investiga- 
tions into the natural history of his subject were persistent, 
and he never ceased to deprecate the ‘‘slop-work” so pain- 
fully evident in the writings of some entomologists. For 
many years he made almost annual excursions in pursuit 
| of his favourite science, especially in the Alps of Central 
