366 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 31 



tant earthquakes, but gives useful records of earthquakes at moderate 

 distances, as in the Imperial Valley and elsewhere in southern Cali- 

 fornia and adjacent Mexico, so that the records are useful in connec- 

 tion with the special investigations being made in southern 

 California (pi. 3, fig. 2). 



Galvanometric recording is used in the Wenner seismometer, 

 developed by Dr. Frank Wenner, of the Bureau of Standards (pi. 2, 

 fig. 1). This instrument has been fully described in publications of 

 the Bureau of Standards - available to anyone interested. It is oper- 

 ated at high magnification and gives exceptionally good records, ex- 

 cept when microseisms interfere with interpretation of the records. 

 Microseisms are small oscillations set up at times in the earth's crust, 

 and, according to various investigators, are to be attributed to effect 

 of breakers on the coasts, and, more probably, to passing areas of 

 considerable range in barometric pressure. They may last for several 

 days and in the case of instruments of high sensitivity may make it 

 impossible to interpret a superimposed earthquake record. These in- 

 struments are installed at San Juan, P. R., at the observatory of the 

 Coast and Geodetic Survey, at the Cathedral of Learning at Pitts- 

 burgh, Pa., and will soon be installed at the station of the Massa- 

 chusetts Institute of Technology near Machias, Me., and at the ob- 

 servatory of the Coast and Geodetic Survey at Sitka, Alaska, and also 

 in the new Franklin Institute Museum in Philadelphia. 



The McCorab-Romberg tilt compensation seismometer has tilt 

 compensation in effective form (pi. 1, fig. 2). The instrument 

 has been developed by H. E. McComb, of the Coast and Geodetic 

 Survey, using a principle developed by Arnold Romberg, now of 

 the University of Texas, but at the University of Hawaii when 

 he did the work (pi. 4). The instrument has photographic record- 

 ing with relatively simple and compact optical system, oil damp- 

 ing, and magnification of about 150. Its distinctive feature is oil 

 coupling between boom and recording mirror. The mirror, which is 

 horizontal, has a vertical stem or shaft which carries a vane. The 

 vane is free to move in a long rectangular container of castor oil, 

 which is attached to the steady mass. The mirror is pivoted on a 

 horizontal axis so that it tilts with every movement of the vane. All 

 rapid movements are transmitted from liquid to vane as if the con- 

 nection were rigid. If, however, owing to tilt, the boom takes a new 

 position, the vane drifts through the liquid with the mirror remaining 

 horizontal. Since the tilt and drift are both very slow, the record is 

 not affected. 



» Wenner, Frank, A now seismometer, etc., Research Paper No. 66, reprint from Bureau 

 of Standards Journ. Research, vol. 2, May, 1929. 



