36 



REPORTS ON THE STATE OF SCIENCE. — 1916. 



case help the determination of epicentre, and we need not draw these 

 arcs. The others will clearly not give arcs meeting in a point, but 

 may be drawn for trial. If the globe is of such material that pencil- 

 marks can be made and rubbed out, the arcs can be drawn on the 

 globe. Or a small piece of thin paper may be attached temporarily 

 to the globe in the neighbourhood of the epicentre — a plan which 

 allows the diagram of the arcs to be preserved for reference. 



It may further be worth remarking that the time at origin can be 

 found without using any tables at all, owing to the fact that the times 

 for S ai'e to those for P very nearly in the ratio of 180 to 100, which 

 happens to be the ratio of the Fahrenheit and Centigrade thermometer 

 scales, and is thus readily retained in the memory. Hence the value 

 of may be calculated thus: — 



The final is got by subtracting from P the sum of S— P and its 

 fourth part. 



But there are certain inconveniences in using a globe, and, indeed, 

 no large enough globe may be available. The stereographic method of 

 projection has been in such cases found very convenient. (It was 

 apparently proposed for this purpose in 1911 by Dr. Otto Klotz, as 



Fig. 1. 



noted below ; possibly also by others, as the device is well known.) It 

 is a property of this projection that all circles on the globe project into 

 circles, though they_ are generally excentric to the projection of the 

 centre. Thus the circle on the globe with centre N (the observing 

 station) and radius A will be represented on the flat projection by a 



