650 



NATURE 



[May 20, 1922 



Directive Radio-telegraphy and Navigation. 



IN foggy weather, sound-signalling stations have 

 proved useful as an aid to navigation. The 

 sounds heard, however, cannot be trusted to give 

 accurate indications either of the distance or direction 

 of the station. Their range also is very limited. It is 

 not surprising, therefore, that many suggestions have 

 been made for utilising the electric waves used in radio- 

 telegraphy to enable a navigator to find his bearings. 

 The propagation of electric waves is unaffected by fog 

 and, unlike sound or light waves, can be transmitted 

 to any distance. Moreover, the apparatus required for 

 radio-signalling is very cheap, requires little skilled 

 attention, and can easily be installed in lighthouses and 

 lightships. Until two or three years ago the radiophares 

 — or radio-beacons as they are called in America — were 

 purely stations for giving ships their positions. In order 

 to find its bearings a ship must send a message to two or 

 more stations, and its direction is located by direction- 

 finding coils. The stations then communicate with one 

 another and so, by the help of triangulation, find the 

 position of the ship, which is communicated to it by 

 radio-telegraphy. In practice the whole operation 

 takes about five minutes. The most extensive chain of 

 direction-finding stations is that controlled by the 

 United States Navy. There are at least thirty stations 

 on the Atlantic seaboard and several on the Pacific 

 coast. France has about ten radiophares and this 

 country has six. A drawback to the method is that 

 valuable time may be lost in getting into communica- 

 tion with the radiophares and in getting the information 

 back to the ship. 



A new and very promising method has been recently 

 developed by the Bureau of Standards at Washington 

 in co-operation with the Bureau of Lighthouses. In this 

 method lighthouses and lightships the locations of which 

 are accurately shown on sailing charts are equipped 

 with radio fog-signalling apparatus. A direction-finder 

 operated by the navigating officer is installed in the ship. 

 It is then easy to find the directions of the various 

 radiophares within his range and thus work out his 

 position on the chart. The results obtained by this 

 method have been very successful, and it seems to be 

 much preferable to the ordinary method of using the 

 direction-finder at the fixed stations. It seems prob- 

 able that every important lighthouse in America will 

 soon become a radio fog-signalling station. The Bureau 

 of Standards suggests that radiophares should be divided 

 into three classes. The first, or long-range class, has a 

 radius of action of 300 miles. The second, or short- 

 range class, can signal to 30 miles ; and the third class 

 comprises the lightship stations which can signal to 

 10 miles. 



The method has been made possible by the perfecting 



of a new radio direction-finder. The principle on which 

 it acts is that the signals received by a flat coil have 

 their maximum intensity when 

 the direction from which they 

 come is in the plane of the coil. 

 When certain precautions are 

 also taken in arranging the 

 apparatus, the signals are of 

 practically zero intensity when 

 the plane of the coil is per- 

 pendicular to the direction 

 from which they come. As 

 Fig. I shows, it is designed to 

 be installed over the ship's 

 binnacle carrying the magnetic 

 compass. The radio-bearings 

 are read directly on the mag- 

 netic compass card. An addi- 

 tional scale, marked with the 

 corrections obtained when 

 calibrating the instrument, is 

 attached to the top of the 

 binnacle so that the true 

 reading can be obtained at 

 once. When taking a bearing 

 the only operation necessary 

 is to rotate the direction-find- 

 ing coil until the sound is a 

 minimum. The ordinary type 

 of direction-finder for use on 

 shipboard consists of a coil of 

 ten turns of insulated copper 

 wire wound on a wooden frame 

 four foot square, which is 

 mounted so that it can be 

 rotated about a vertical axis. 

 Suitable receiving apparatus 

 is used in connection with this 

 coil, namely, a variable air 

 condenser for tuning purposes, 

 a six-tube amplifier having 

 three stages of radio-frequency 

 amplification, a detector, two 

 stages of audio-frequency 

 amplification, batteries, and 

 suitable telephone receivers. 



The Bureau of Standards 

 have issued a pamphlet by 

 F. A. Kolster and F. W. Dun- 

 more giving a full description 

 of the direction-finder, point- 

 ing out some of the difficulties that had to be overcome 

 in developing it, and giving many experimental results. 



At m 



F13. I.— Magnetic compass with 

 direction-finder attachment 

 for reading the radio-bearing 

 directly on the magnetic 

 compass. 



The Cause and Character of Earthquakes.^ 



By R. D. Oldham, F.R.S. 



THE study of earthquakes, using that word in the 

 restricted, and original, sense of the disturbance 

 of the ground which is sensible to human feelings, which 

 causes alarm and destruction, and is properly that seism 



^ Abridged from the presidential address delivered before the Geological 

 Society of London on February 17. 



of the ancient Greeks, from which our modern term 

 seismology is derived, has always been recognised 

 as one of the departments of geology. This 

 limitation is necessary, for, of late years, seismology 

 has been extended to the study of a phenomenon of 

 different character, the long-distance records of dis- 



NO. 2742, VOL. 109] 



