
Se 
May 19, 1923] 
NATURE 
a 
673 

of petrol per brake horse-power per hour, and it is 
probable that an airscrew efficiency of 75 per cent. 
may be reached. With petrol weighing about 7 lb. 
to the gallon it will be found from those figures and a 
weight-resistance ratio of 9 that the number of miles 
possible per gallon of petrol consumed is roughly equal 
to thirty thousand divided by the weight of the aero- 
plane. To achieve roo miles to the gallon, therefore, 
it would be necessary to improve on existing figures 
for performance, since a gross weight of 500 lb. yields 
only 60 miles per gallon. As an aero-taxi with pilot and 
passenger would weigh more than 500 lb. it would appear 
that M. Barbot’s estimate is not to be easily achieved. 
It will be noted, however, that the condition of a 
minimum of so miles per gallon in the competition 
rules for the Duke of Sutherland prize is reasonable. 
It will exclude seriously inefficient design without 
setting an impossible task. 
In making the preceding calculations, no allowance 
was made for the use of energy obtained from the 
wind itself. As we all know, aeroplanes without 
engines, z.e. gliders, have maintained themselves in 
the air for several hours consecutively, utilising winds 
deflected upwards by a sloping hill-side. Up-currents 
of sufficient intensity for support are very local, and 
we are yet far from being able to use them for point- 
to-point journeys as distinct from tacking backwards 
and forwards in a chosen locality. The phenomenon of 
gliding, as we know it, does not modify the estimate 
of power already made, but does show how part of that 
_ power may be obtained from the wind. Langley con- 
templated the extraction of energy from the pulsations 
of the wind, quite apart from their direction, and this 
source of energy is probably very widely distributed. 
So far, however, little, if anything, has been attempted 
in practice in the extraction of this energy, and there 
is no clear lead as to the direction in which one might 
hopefully proceed. 
For some time to come, it may be expected that the 
-aviette will carry the main source of power for its 
support ; economy of fuel may be obtained by utilising 
-up-currents in the neighbourhood of flight and so 
using the engine in passing from hill crest to hill crest. 
The condition attached to the Duke of Sutherland 
prize that the competition is to take place over a 
triangular course of not less than 15 miles reduces 
the chances of obtaining substantial amounts of energy 
from the atmosphere. 
Since a claim for reduction of risk has been made, 
it may be as well to state the view that the only 
contribution made to safety is in the sense that it is 
less dangerous to strike the ground at 30 m.p.h. than 
at 60 m.p.h. The inherent defects of the modern 
aeroplane which make for danger on stalling are quite 
untouched by the new application. ~ 
The low-power aeroplane can scarcely fail to react 
beneficially on scientific knowledge and its applications. 
At the moment, however, it would appear that the 
aviette has derived its being from knowledge obtained 
for other purposes and has not reached the stage of 
reciprocation. 
[Since this article was written, a very definite 
advance in the practica] use of the low-power aero- 
plane has been made by M. Barbot, who, as was 
recorded in our issue of May 12, p. 645, succeeded in 
completing the round trip from the French coast to 
Lympne in Kent and back again, covering about 80 
miles, in a machine with a 15 h.p. engine. His time 
for the journey was about two hours and a quarter, 
including nearly half an hour’s stay at Lympne, and 
it is stated that his petrol consumption was about 
two gallons.] 
The Earth’s Electric and Magnetic Fields.* 
By Prof. W. F. G. Swann, University of Minnesota. 
Il. 
WO of the most characteristic features of the 
‘earth’s magnetism are the non-coincidence of 
the magnetic and geographic axis, and the secular 
variation. While a theory which is to claim any degree 
of completeness must account for these, one cannot 
resist the temptation of searching for any sort of 
phenomenon capable of giving a field of the order of 
magnitude of the earth’s field in the hope that if such 
be found it may serve as a possible basis on which to 
build a more complete theory. 
Attempts towards a theory of the earth’s magnet- 
ism may be classed roughly in the following four 
groups : 
(rt) The earth is assumed to be made largely of iron, 
and to be a permanent magnet independently of its 
rotation, or to be magnétised inductively by an external 
field. 
(2) The magnetic field is brought about by the rota- 
tion of an electrostatically charged system. 
(3) The magnetic field arises from a state of magnet- 
isation brought about by the rotation of the earth. 
1 Continued from p. 642. 
NO. 2794, VOL. 111] 
(4) The field is caused by electric currents circulating 
within the earth. 
The high temperature of the earth’s interior would 
be inconsistent with a state of permanent magnetisation 
unless the effect of high temperature is compensated in 
some way by that of high pressure. Induced magnet- 
isation suffers from the same cause, and would, more- 
over, give a type of field totally unlike the earth’s field. 
As regards (2), a sphere of the earth’s size, rotating 
with the earth’s angular velocity, would have to possess 
such a surface charge as would give it a potential 
gradient a hundred million times the earth’s potential 
gradient in order that the rotation of that surface charge 
would produce a magnetic field comparable with the 
earth’s magnetic field. Even then, it turns out, as 
has been shown by Schuster and by the writer, that 
owing to the effect of the observer’s motion with the 
earth’s surface, a sign of charge which gave the right 
direction for the vertical component would give 
the wrong direction for the observed horizontal 
component.* 
2 Inclusion of the effect due to the atmospheric positive charge annuls 
completely the magnetic field which would be observed by one moving with 
the earth. 
