SEISMOLOGY. 393 



took the construction, in the shop of the Institute, of the instrument which was 

 planned. 



After several conferences these gentlemen agreed upon the desirability of 

 proceeding along two lines: (1) to build a sensitive vertical seismograph for 

 the measurement of earth displacements during shocks; (2) to design a sensi- 

 tive accelerograph for the measurement of the acceleration of movement. 

 These two sets of instruments are now under construction in Pasadena. 



The general plan of procedure for this phase of the investigation assumes 

 that the earth faults will give evidence of their activity by occasional slips 

 which will cause disturbances to travel outward from points of structural 

 weakness as wave pulses. To obtain definite information concerning the loca- 

 tion of the sources and the direction of movement of these slips, it is considered 

 necessary to record accurately the motions which find expression in these wave 

 pulses and the relative times at which the pulses arrive at two or more observ- 

 ing-points. The requirements for such instruments are, first of all, sensitive- 

 ness and reliability to a degree not obtainable from seismometers, such as are 

 now in use in the usual teleseismological observation stations. The plan for 

 one of these sets of apparatus is patterned somewhat after that of Prince 

 Galitzin, in which use is made of the usual pendulum mounted so as to have 

 a period of the order of 10 seconds, recording photographically on moving 

 sensitive paper the beam of light reflected from a galvanometer mirror. The 

 galvanometer is connected to a coil of insulated wire attached to the pendulum 

 and placed in the field of a strong magnet fixed to the pendulum support. 

 When there is relative motion between the pendulum and its support an 

 electromotive force is developed in the circuit which gives a current and 

 consequently a deflection of the moving element of the galvanometer. In 

 this system the deflections are arranged to be approximately proportional 

 to the relative displacements of the pendulum and its support, that is to say, 

 the earth at the point of observation. The present design is planned to record 

 pulses having effective periods varying from 0.05 to 2.0 seconds, and any 

 magnification desired up to 500. It is hoped that a field test of this instrument 

 can be made before the close of the present calendar year. 



5. Soundings. 

 Through the cooperation of the Navy Department, the Hydrographic Office 

 has arranged to equip two destroyers with appropriate sounding devices and, 

 as soon as they are ready, to undertake an elaborate network of soundings 

 from Los Coronados at the Mexican boundary to Point Conception, and per- 

 haps farther north if the results warrant it. This region is opposite the great 

 fault zone of southern California and the determination of the faults there will 

 complete our information regarding lines of structural weakness in the region 

 to which the most intensive study is proposed to be given. These soundings 

 will begin about the middle of October 1922 and are expected to yield a more 

 complete map of the undersea configuration than has ever been developed 

 hitherto. 



6. Isostasy. 



The problem of isostatic adjustment and its possible relation to crustal 

 movement depends, at the moment, primarily upon the possibility of obtaining 

 a very much greater number of well-distributed measurements of the constant 

 of gravitation than are now available. But this must also await the design 



