May I'], 1922] 



NATURE 



687 



in our very fragmentary knowledge of what goes on 

 within the substance of the earth, we have means of 

 explaining and interpreting the greater part of the facts 

 known to us regarding the character of earthquakes. 

 I shall, therefore, conclude by summing up the con- 

 clusions which have been put forward as to origin and 

 cause. These are, first, that earthquakes are not due 

 to any slow-acting process of secular duration, but to a 

 rapid development of strain, which may, in extreme 



cases, be almost instantaneous — a conclusion which I 

 believe to be true of the greater part at least of those 

 usually classed as tectonic, and of all those of great 

 magnitude ; and, secondly, that the development of 

 strain is not the result of processes which have pro- 

 duced the tectonic structures, recognised by surface 

 observation, but of changes and displacements in the 

 matter which lies below the cooled and solid outer 

 crust. 



Telegraphic Transmission of Photographs. 



A NUMBER of experimenters have attained varying 

 measures of success in solving the problem of 

 transmitting photographs, drawings, handwriting, etc., 

 by line and wireless telegraphy, and a good deal of 

 attention has been directed recently to the latest 

 developments of the system on which M. E. Belin has 

 been working in France for some time. His apparatus 

 has been used with good results between the large 

 French wireless station near Bordeaux and a naval 

 station in the United States, as well as over land 

 telephone circuits, etc. A brief description of the 



included in a suitable circuit, arranged so that a current 

 of varying strength is produced, owing to the variations 

 of the resistance of the microphone according to the 

 thickness of the part of the film that is being passed 

 over. This var}'ing current can either be sent directly 

 ov'er the line, or can be employed to control the strength 

 of the waves sent out, in the case of wireless trans- 

 mission. 



The manner in which the variations in the signal 

 current, or wave train, are retranslated into a photo- 

 graph by the receiving apparatus is scarcely more 



Portable apparatus for the telegraphic transmission of photographs. 



latest form of the apparatus appeared in the Comptes 

 rendus of the Paris Academy of Sciences of March 6, 

 from which the accompanving illustration is reproduced 

 (Fig. I). 



As in most of the experimental systems of " tele- 

 photography," sjTichronously rotating drums are made 

 use of in the sending and the receiving apparatus 

 respectively, with a simple arrangement of correcting 

 signals to keep them in step. Mounted on the cylinder 

 of the sending apparatus is a print of the photograph 

 to be transmitted, made on a special bichromatised 

 film which gives an image in appreciable relief. It is 

 not necessary, however, to metalise this image to render 

 it conducting, as is necessary in some systems, and it 

 will be seen that the use of selenium cells, which 

 forms a feature in other systems, is also avoided. 

 A stylus, in a holder which is given a slow axial feed, is 

 caused to pass over all portions of the relief film in 

 succession, after the manner of the needle of a phono- 

 graph. This stylus is attached to the diaphragm of a 

 simple but sensitive form of carbon granule microphone, 



NO. 2743, VOL. 109] 



complicated. The varying current from the line (or 

 the wireless receiving apparatus) is passed through 

 a delicate reflecting galvanometer such as a Blondel 

 oscillograph, the mirror of which is deflected by an 

 amount depending on the strength of the current, i.e. 

 on the thickness of the film where it is being passed 

 over by the stylus of the transmitting apparatus. The 

 light from the mirror passes through a screen of 

 graduated capacity, and the optical system is arranged 

 so that an image of the mirror, varying in brightness 

 according to the deflection, is projected on to a photo- 

 graphic film on the drum, which is moving synchron- 

 ously with that in the transmitting instrument. The 

 photographic effect produced at any point is therefore 

 always proportional to the thickness of the original 

 film, so that a duplicate photograph formed of a screen 

 of fiine lines, but with a full range of " half-tones," is 

 produced. In a simpler form of the apparatus, for 

 pure black and white or " line " work only, a contact- 

 maker replaces the variable resistance microphone in 

 the transmitting apparatus, and a diaphragm, which 



