«ECTioN m., iyo&. [ 197 ] Tkans. r. s. c. 



XI. — Microseisms. 



By Otto Klotz, liL.D. 



Presented by Dr. W. F. King. 



Under this designation are included all pulsations not directly 

 attributable to what are generally known as earthquakes, which give 

 abrupt, more or less violent, and momentary movements of the earth's 

 crust, the effect of which may, how3ver, continue for some hours. At- 

 tempts have been made to classify these microseisms according to their 

 cause, but so far without complete success. During the past year the 

 writer has paid considerable attention to these disturbances, and in 

 doing so has studied and compared the daily scismograms with our 

 daily microbarograms, our weekly aneroid barograms and also with the 

 daily weather maps, wliich give the isobars at 8 A. M. for Canada and 

 the United States, roughly between latitudes 25° and 55°, and the 

 Atlantic and Pacific Oceans. The microbarogram exhibits particularly 

 well very rapid fluctuations of pressure as manifested by local and often 

 gusty winds, the counterpart of which is always represented: on the 

 seismogram by irregular movements and not by microseisms. The 

 average time of the beginning of the seismogram sheet is about 10 

 A. M., so that the above isobars and gradients dependent upon them 

 are for a time preceding the former by two hours. From the examin- 

 ation of the local barogram alone not much information can be gathered 

 as to the behaviour of the seismograph except when very rapid and 

 marked fluctuations, say a millimetre or more, take place in the pres- 

 sure, comparable with the "' pumping " of a mercurial barometer at sea. 

 The 'barometer may show little or no change in pressure at a given place 

 yet areas of "High"' and '"Low" (barometer) may be rusliing along 

 to the north and south of it, setting up vibrations or pulsations of the 

 earth's surface that may be markedly felt at a given place by the seis- 

 mograph. Similarly from a large rise or fall of the barometer during 

 24 hours at a given place alone, we can draw no gradients to determine 

 the atmospheric movements ; the position of the isobars and " Highs " 

 and " Lows " being unknown. We have simply the record of the vertical 

 movement of pressure at one point. The isobars on the Weather Map 

 are drawn at intervals of one-tenth inch difference of pressure. The 

 normal to the isobars is called the gradient, and when spoken of, gener- 



Sec. III., 1909. 12. 



