8386 
NATURE 
[JUNE 30, 1923 

thought to the expert who devises it, it may demand 
such a mental strain of the general chemical reader 
as to defeat a main purpose for which it was planned, 
namely, the communication of knowledge. I would 
quote, both with regard to chemical symbolism and 
to chemical nomenclature, the words used by a character 
of Henry James concerning literature, “‘ The observer 
is nothing without his categories, his types and 
varieties. . . . That’s for his own convenience ; he 
has privately a terminology to meet it. . . . But from 
the moment it’s for the convenience of others, the 

signs have to be grosser, the shades begin to go. . 
Literature, you see, is for the convenience of others. 
It requires the most abject concessions.” Scientific 
literature is, above all, for the enlightenment and 
convenience of others, and scientific specialists must 
be prepared to make concessions to their weaker or 
less expert brethren. But whether the symbols we 
use are simple or complicated, we should always be 
clear as to their true significance, and be on our guard 
against their distracting our thoughts from the realities 
which they partly reveal and partly obscure. 

Recent Contributions to Aviation Problems. 
By Prof. G. H. 
ee attempts which have hitherto been made to 
explain the continuous sustentation of tropical 
and other birds without the performance of muscular 
work have left many doubtful points requiring to be 
cleared up. Observers have frequently sought to 
attribute the phenomena to something acting in viola- 
tion of the principles of elementary mechanics, and 
have succeeded in establishing this peculiarity, not 
perhaps in the way that they intended, but by the 
chaotic way in which such terms as force, momentum, 
weight, energy, lift, pull, drag, and gravity are con- 
fused by them, and occur indiscriminately mixed up in 
their writings. In a paper on “ Meteorology and the 
Non-Flapping Flight of Tropical Birds,” published in 
the Proceedings of the Cambridge Philosophical Society, 
xxi. 4, Dr. Gilbert T. Walker has now sought to bring 
sound scientific principles to bear on the numerous 
observations in India published by Dr. Hankin. From 
observations of the temperature of the air at Agra at 
different altitudes and hours of the day, Dr. Walker finds 
conditions of instability leading to the formation of - 
strong ascending air-currents,and observations in Egypt 
and various parts of India indicate conditions of ‘‘ bumpi- 
ness” in the atmosphere caused by ascending currents, 
covering the periods employed by birds for “‘ soaring.” 
Dr. Walker finds that the angle of gliding of the most 
efficient recent aeroplanes is sufficiently low to satisfy 
the conditions necessary for continuous sustentation in 
the presence of these currents, and he examines in 
detail three possible sources of internal work in the 
atmosphere, namely, ascending currents, variations of 
horizontal velocity as functions of time and place 
co-ordinates, and Lord Rayleigh’s hypothesis of varia- 
tion of horizontal wind velocity as a function of the 
vertical altitude co-ordinate. He also carefully considers 
the structure of the birds’ wings, in comparison with 
those of the Handley-Page machine, and the effect 
of the flexibility of the quills on the aerofoil form in 
ascending and descending glides. Both from theory 
and from actual observation, it is found that the 
ascending air-currents in the higher regions of the 
atmosphere are greater in diameter than lower down, 
and from actual observation he contradicts Dr. 
Hankin’s statements according to which birds are seen 
rising in descending currents. It would thus appear 
from Dr. Walker’s observations that, in the region 
dealt with by Dr. Hankin, the atmosphere possesses 
sufficient internal energy to satisfy the conditions of 
“soarability ’? required by the latter observer. In 
NO. 2800, VOL. 111 | 

Bryan, F.R.S. 
regard to the violation of mechanical principles, both 
hypothetically by birds and actually by writers, we 
cannot do better than quote Dr. Walker’s remark that 
5 “it is strangely” (asterisk and footnote with 
references follow) “necessary to insist that it is as 
impossible to derive energy from a wind that is constant 
in time and space as it is from a perfect calm.” To 
theories based on a denial of this principle the late Sir 
Hiram Maxim used to point out the enormous velocity — 
of the wind due to the earth’s rotation and its orbital 
motion about the sun, and he suggested that if writers 
believed in these theories, why did not they utilise this 
energy for the purposes of flight ? 
The recent records of gliding flight in the daily press — 
afford ample confirmation of Dr. Walker’s theories in 
regard to the quantity of available internal energy 
present in the atmosphere. When we read of aviators 
remaining for hours in the air under climatic conditions — 
not widely different from those prevailing in Dr. 
Hankin’s and Dr. Walker’s investigations, and of 7 horse- 
power engines making long flights at a cost for fuel of 
not more than a penny a mile, we have reasonable 
prospects of realising a system of cheap transport 
largely superseding the use of wheels bumping over 
stony roads or iron bars placed end to end. 
Apart, however, from the precariousness of the dis- 
tribution of the necessary internal energy in space and 
time, a motorless aeroplane is in constant danger of 
being suddenly brought to rest relative to the air, or, 
more accurately, losing all headway, at which instant it 
has a vertical acceleration due to gravity, and the 
resistance to rotation (pitching, yawing, rolling) 
becomes technically describable as “a small quantity 
of the second order,” thus approximating to the con- 
ditions assumed in the problem of rigid body rotation 
under no forces. 
We are constantly reading of accidents which seem 
to suggest that they have arisen from this condition of 
affairs, even in the case of motor-driven machines 
which are at least equipped with a more adequate means 
of extricating themselves from this eventuality. 
No system of aviation will ever be satisfactory, 
however, until backed up by a more thorough solution 
than we now have of the equations of motion of the 
“perfect aeroplane.’ Perfect fluids, perfect conduc- 
tors and dielectrics, perfectly smooth bodies, perfect 
gases, and so on, are very familiar terms, but the 
“perfect aeroplane ” has not yet figured in literature as 
such, though various formal representations have been 

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