a i i i ee 

May 26, 1923] 
NATURE 
709 
Nene ene _________.. 
survey could be successfully carried out without it 
when working from pushers. When flying without this 
apparatus, either more skill is required, or the beginnings 
of each strip will be rather less accurate than they 
might be. 
The photographic strips themselves can be kept 
straight by flying on a distant point near the horizon, 
but this operation can be much assisted by another gyro 
instrument that controls the rudder through a relay 
and thus keeps the aeroplane on a straight course auto- 
matically. This apparatus relieves the pilot of the 
most fatiguing part of his work and, by allowing him to 
concentrate more on such things as maintaining constant 
height and speed, improves the general quality of his 
work. We have carried out surveys both with and 
without this instrument, and, while we have proved 
‘that accurate work can be done without it, we should 
always recommend its use in any large surveying 
scheme. 
AREA COVERED IN A FLIGHT. 
We have found from experience that roo sq. miles is 
about the area that can conveniently be covered in one 
flight. This requires about 80 minutes flying on the 
actual mapping and about three hours from ground to 
ground. This amount of work is about what a crew 
can perform regularly, day by day: hence it follows 
that aerial surveying by these methods can be carried 
out at the rate of about 100 sq. miles per day. If the 
separate strips are made ten miles long, the average 
day’s work will, therefore, cover a square of ten miles 
to the side. 
We have found that an area of too sq. miles, involv- 
ing about 130 photos, forms a convenient unit for com- 
pilation, for, although we have compiled avery successful 
map of 225 sq. miles in one unit, we consider this to be 
too large for economical work. The method, therefore, 
that we favour for mapping large areas, is to compile 
the prints of each day’s work into separate mosaics and, 
after reproducing these to any required scale in a large 
camera, or photostat, to fit these larger units together 
in the same way as the individual prints were fitted. 
(To be continued.) 
Science and Radio-Communication.' 
By Sir Ricuarp GiazeBprook, K.C.B., F.RS. 
ROBLEMS in which there is a close connexion 
between theory and practice can be found in 
every branch of engineering, perhaps with more striking 
effect in electrical and metallurgical science, in the laws 
of stress and strain in structural materials, and in the 
fatigue of parts subject to vibration, rather than in 
the questions which pertain more closely to the domain 
of civil engineering. Let me deal first, briefly and 
incompletely it must be, I fear, with that branch of 
electrical engineering—radio, or wireless telegraphy— 
which at present exercises such a fascination over the 
popular mind, which is already and will be to a greater 
extent in the future a link to bind together all nations 
of the earth. Sir William Anderson, in the first James 
Forrest lecture delivered thirty years ago, refers to 
Preece’s early experiments between Lavernock and 
Flatholme, a distance of eight miles, as a startling 
consequence of electro-magnetic theory. Now the 
earth is girdled with a wireless chain depending from 
two, or at most three, great stations. I have just 
received from the International Union for Scientific 
Radio Telegraphy details of a scheme for the determina- 
tion of longitude in which the principal co-operating 
stations will be Bordeaux, Annapolis, and Pearl 
Harbour. 
In the year 1865 Clerk Maxwell read before the Royal 
Society his paper on “ The Equations of the Electro- 
Magnetic Field.” It was an attempt, which has stood 
the test of time—the conditions which led Lorentz and, 
later, Einstein to introduce certain modifications were 
not dealt with by Maxwell—to apply mathematical 
reasoning to those principles, enunciated by Faraday, 
on which the construction of generators and motors, 
transformers, and practically all electrical machinery 
is based. This reasoning led him to the result that the 
effect of changes in an electric current in a conducting 
1 From the James Forrest lecture on ‘“* The Interdependence of Abstract 
ag and Engineering,” delivered before the Institution of Civil Engineers 
on May 4. 
NO. 2795, VOL. III] 
wire would be propagated through space with a speed 
depending on the two constants* which define the 
electric and magnetic conditions of the medium sur- 
rounding the wire. The values of these constants for 
air can be found from electrical considerations, and 
hence the velocity with which electro-magnetic dis- 
turbances are propagated can be calculated. To quote 
his words : 
“We now proceed to investigate whether these pro- 
erties of that which constitutes the electro-magnetic 
eld, deduced from electro-magnetic phenomena alone, 
are sufficient to explain the propagation of light 
through the same substance,” and his conclusion is: 
“The agreement of the results seems to show that 
light and magnetism are affections of the same 
substance and that light is an electro-magnetic dis- 
turbance propagated through the field according to 
electro-magnetic laws.”’ 
Maxwell found that when the calculations were made 
the resulting value for the velocity was approximately 
equal to the velocity of light. The work was extended 
in his “Treatise on Electricity and Magnetism,” 
published in 1873. The values of the velocity of light 
and the velocity of propagation of electro-magnetic 
waves were not known then with present-day accuracy, 
and he concludes that they are quantities of the same 
order of magnitude. A glance at present-day ® figures 
shows that they are identical, and the electro-magnetic 
theory of light is universally accepted. Nor was the 
result true only for propagation through air or inter- 
stellar space ; such observations as were then available 
showed that, in all probability, it held for all transparent 
media, though there were discrepancies, known now to 
2 The velocity is given by 1/ al pk, where & is the inductive capacity and 
yw the magnetic permeability of the surrounding medium. ’ : 
3 Messrs. Rosa and Dorsey of the Bureau of Standards, discussing the 
various determinations of the electro-magnetic velocity, express the view 
that the figure 2:9980x 10" cm./sec. is accurate to Ir part in 10,000, 
while the best result for the velocity of light is, to the same accuracy of 
measurement, 2:9986™ 10! cm./sec. See ‘‘ Dictionary of Applied Physics,” 
vol. ii. 
