74 



DISCOVERY 



versely, the other longitudinally. The wires leading 

 from these frames end on a switch-board containing 

 all the electrical equipment. Each is connected to a. 

 telephone, and observations are made by listening in. 

 The ship is manoeuvred to enter a harbour safely as 

 follows : The ship is first steered approximately by 

 soundings. It b?ing known that the guiding cable lies, 

 say, in a direction from east to west, a course north- 

 west must be set to pass over it. The navigator 

 naturally does not know at what point he will pass over 

 the cable, but pass over it he must. An observer is 

 placed at the switch-board and listens in on the trans- 

 verse frame with sixty turns. At a given moment he 

 receives a signal. The ship is then so set as to receive 

 a maximum of waves from the cable on the transverse 



Fig. I.— HORIZONTAI, 0SCII,I,ATING FRAME. 

 This enables an observer to study the aerial field above the earth, and to 

 determine the direction of the source emitting the electric waves. 



and a minimum on the longitudinal frame, indicating 

 that the course is at right angles to, and of course 

 approaching, the cable. The intensity of reception 

 increases as the cable is approached. From time to 

 time the observer listens in on the spiral of ten turns 

 of the transverse frame, but hears nothing until the 

 ship is sufficiently near. At the moment of making 

 contact on this coil the vessel is turned through 90'", 

 for this signal indicates that it is about 300 metres from 

 the cable. The ship is then set so that there is a 

 good signal on the small longitudinal frame and zero 

 signal on the transverse. In this case a course is being 

 held rigorously parallel to the cable and at a distance 

 less than 300 metres from it. Even if there is a strong 

 cross-current, it is possible to maintain on the longi- 

 tudinal frame a signal of constant intensity. Vessels 



keeping to the right — i.e. having always the cable on 

 their left — know its direction and consequently their 

 own course. At any time they can find out whether 

 they are approaching or receding from the cable and 

 know where they are relatively to it. Thus the dangers 

 of fouling, of grounding, or of striking a rock disappear 

 completely, and make navigation practicable. It was 

 found possible, for example, to steer a 400-ton gunboat. 

 La Belliquense, along a cable placed at the mouth of 

 the entrance to the Brest harbour under conditions of 

 perfect safety. La Gloire, a 10,000-ton cruiser, was 

 also piloted along the same cable. 



The current flowing through the submerged cable 

 is only 2'5 amperes. Yet it can be detected, even 

 without adjustment to resonance, as far away as two 

 to three kilometres, so that the total listening zone is 

 four to six kilometres. 



It may be added that these experiments, witnessed 

 by the Minister of Marine, were first conceived in the 

 laboratory in 1914, but not carried out until little 

 more than eighteen months ago with the help and 

 co-operation of the Dredging Commission of the French 

 Admiralty. The method, based on a number of 

 patents, was not announced until much later, not, in 

 fact, until the inventor was fully satisfied with its 

 efficiency. 



Shortly afterwards M. Loth discovered that in the 

 case of aviation at sea the total listening zone in the 

 air above the submerged cable was three kilometres. 

 It was this which suggested his second discovery, 

 which arose from a further series of experiments on the 

 problem of guiding the nocturnal flight of aeroplanes 

 and airships. 



On the Villacoublay aviation ground is an old 

 interred cable, formerly used for carrying an electric 

 current, which zigzags no fewer than eight times and 

 underlies much of the area of the ground. By means of 

 this cable, after it had been put into a state of repair, 

 M. Loth experimented with his receiving apparatus, 

 installed first on Farman aeroplanes and afterwards 

 on Nieuport machines, placed at his disposal by M. 

 Leon Bazaine and the Nieuport Aeroplane Company. 

 The inventor began by making a complete study of 

 the magnetic field set up when an alternating current 

 with a frequency of 600 per second was passed through 

 the cable. This field was explored at great distances 

 from the cable (at one time a distance of several 

 kilometres, with the receiving apparatus on a motor- 

 car), and often yard by yard. M. Loth also studied 

 the exact form of the magnetic fields of the magnetos 

 on his aeroplanes, which at first sight would appear 

 to complicate the problem. 



This difficulty, however, was overcome, and the noises 

 from the magnetos were almost entirely suppressed, 

 and did not interfere with the reception of the waves 



