408 
each artillery battery must be in unbroken 
communication with the brigade headquarters, 
and the division and staff officers. Wireless 
telegraphy and telephone are used in various 
ways; ordinary telephones are installed every- 
where: carrier pigeons must be bred and 
trained; signals using both sound and light 
are most useful. It is not merely a matter of 
perfecting signals which work satisfactorily; 
much more is required, safeguards must be 
devised which make it impossible for the 
enemy to observe or read them. When I say 
that all this has been done, and done to a 
large degree by our American physicists, I am 
telling only the bare truth. I wish it were 
permitted to tell more. 
_ Signals as used by the Navy are not as 
varied as those required by the Army. But 
there is one special problem which concerns 
the American Navy more than any other. We 
are sending ships and transports to Kurope in 
large groups, as you know, and at night no 
lights are shown by any vessel; the question 
then arises, how is it possible to maintain 
relative distances and positions? This sounds 
as if it were an almost hopeless proposition; 
but it is not; and I have seen a solution which 
seems satisfactory, again the ingenious idea 
of an American physicist. 
The demands upon photography are great, 
largely in connection with airplanes; and the 
methods elaborated by the British and French 
scientists are beautiful. There are other 
phases, though, of almost equal importance. 
Can we not take photographs of objects which 
the eye can not see, owing to clouds, haze or 
distance? This matter is solved in a large de- 
gree as a result of our spectroscopic knowl- 
edge. By photographie methods it is possible 
to discover the location of the enemy’s bat- 
teries unless they are hidden with the utmost 
care. In this last case resort is had, as you 
know, to what is called sound-ranging. When 
a gun ejects its shell in the direction of the 
enemy, the latter hears in succession three 
sounds; first that due to the passing of the 
shell through the air, in general a hissing 
sound; then the proper sound from the gun 
mouth, a boom; and finally the sound of the 
SCIENCE 
[N. S. Vou. XLVIII. No. 1243 
explosion of the shell. Sound waves travel 
through the air with a comparatively slow ve- 
locity, slightly over 1,000 feet per second, and 
so if observing stations are placed at different 
distances from the gun, any one type of the 
three sounds, e. g., the boom, will be heard at 
different instants of time. Jt is easily seen, 
then, that methods may be devised by means 
of a system of triangulation, by which the lo- 
cation of the gun may be determined. The 
accuracy of the methods in use is so great that 
now within a few minutes after the firing of a 
gun its position is known definitely to within 
limits less the accuracy of the guns which are 
responsible for the destruction of the enemy’s 
battery. This last limitation is due to an un- 
avoidable variation in shells and their powder 
charges, and to variations in the atmosphere. 
This method of sound-ranging is simple in 
theory, but extremely difficult in practise, ow- 
ing to vagaries of the wind and to the con- 
fusion caused by simultaneous discharges of 
guns. The former difficulty has been overcome 
by a brilliant British physicist; but, as you 
have probably seen in the papers, one of the 
ways used by the Germans to conceal the posi- 
tion of its big guns by which they were bom- 
barding Paris was to discharge a dozen other 
guns simultaneously. 
A somewhat similar problem arises in con- 
nection with determining the position of an 
airplane at night, or in cloudy weather. One 
inherent difficulty here lies in the great speed 
and great height of the airplane. Rumors 
have reached us that the British have found 
a method; but, whether this is true or not, the 
problem is not hopeless. The airplane in 
flight emits sounds, loud ones; with that fact 
as a basis, its detection is therefore certain. 
I am not sure that any of you would of your 
own account think of astronomy as being a 
practical science; yet there has been found a 
definite usefulness for these disturbances on 
the sun, known as sunspots; and astronomers 
easily turn from calculations of the motions 
of comets, planets and satellites to those of 
twelve-inch shells and bombs dropped from 
airplanes. The instruments used by naviga- 
tors on the sea and in the air when the flights 
