SCIENCE AND ITS APPLICATION TO MARINE PROBLEMS. 
be used by chasing ships travelling at all speeds, and when applied with 
certain restrictions and definite characteristics, it enables one to pick 
up and close on a submarine situated more than a mile away. The 
method, it is obvious, is applicable to the locating of minefields and other 
obstacles to navigation, as well as to submarine chasing. 
(c) Magnetic and Electromagnetic Detection—Magnetic detectors 
usually require the movable system to be poised or pivoted. They can, 
therefore, be used as yet with only a moderate degree of satisfaction. in 
towed bodies or in vessels subjected to violent mechanical disturbances. 
The range at which magnetic effects can be detected is, moreover, com- 
paratively short. As a result of these defects, the use of magnetic 
detection is somewhat circumscribed. Such instruments can, however, 
be used under certain conditions, and in particular sea areas with great 
effect. In the war, very considerable results were actually obtained by 
their use. 
The range at which electro-magnetic detection can be applied is 
greater than is possible with magnetic detection, but the method is, how- 
ever, essentially a short-range one, and in many of the forms in which 
it has been worked out it cannot be used with success at distances 
greater than about 300 yards, or in depths greater than about- 100 
fathoms. 
(d) Leader Gear.—An important application of an electro-magnetic 
effect which was developed during the war is found in what is known as 
Leader gear. This gear consists of a cable laid on the bottom of 
the sea along the course of a narrow tortuous channel leading into a 
harbor or through a minefield. If an alternating electric current be 
passed through such a cable, it is possible, by means of delicate devices 
installed on a ship, to obtain either oral or visual indications of the 
presence of such a cable, and by these indications the ship can be guided 
in safety in fog or darkness at speeds as high as 20 knots almost with 
as much precision as a tramear by trolley wires over a railway. Experi- 
ment has shown that it is a simple matter to apply this method in water 
of suitable depth for distances as great as 50 miles or longer. 
(e) Invisible Signalling—Research has shown that it is possible, 
under certain conditions, to utilize polarized light or ultra-violet and 
infra-red radiations for secret signalling. With the last-mentioned 
type of radiation, especially valuable results are obtainable over con- 
siderable distances, even in the presence of light fogs. | Where it is not 
advisable to use wireless communication between chasing ships, infra-red 
signalling is of special value. : 
(f) Wireless Telegraphy and Telephony.—One of the most remark- 
able developments which have taken place in the war is in the field of 
wireless telegraphy and telephony. ‘By the use of oscillating thermionic 
valves especially great progress has been made. It is now possible to 
hold conversation with ease between a land station or a ship and an 
airship or seaplane over considerable distances, and by this means 
observers on aeroplanes or aircraft can also converse with one another. 
With high-power installations, it has been demonstrated that wireless 
telephonic communication can be maintained on the sea over hundreds 
of miles. : : 
On. the directional side of “ wireless” great advances have also been 
made. If an aeroplane, an airship, or surface ship should send out 
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