J. E. Buvbank — One Phase of Microseismic Motion. 471 



January 31, 1908, were published* in tabular form, with full 

 notes on the atmospheric pressure conditions. These results 

 showed that the most pronounced microseisms were almost 

 invariably connected with the passage of deep lows across the 

 coast line from land to sea, or vice versa. It was also pointed 

 out that the water area under the pressure disturbance would 

 be in hydrostatic equilibrium, while the land area would be 

 subject to a stress which .would be greatest at the shore line, 

 hence we should expect the greatest microseismic motion when 

 the center of a low area moves rapidly over the coast line. 



This reasoning has been confirmed by approximately 100 

 well-defined cases during the past 5 years. In fact, during the 

 period under investigation there has not been a single case of a 

 well-defined low area which has crossed the coast line between 

 Maine and Florida which has not been accompanied by well- 

 defined microseisms. It was also noted in the above paper 

 that a rapid rising or falling pressure over the coast was 

 accompanied by microseisms. 



This type of microseisms has been studied by Dr. Otto 

 Klotz of Ottawa, Canada, who finds that the most marked 

 cases at Ottawa are connected with the passage of low areas 

 down the St. Lawrence and into the Gulf. He considers the 

 microseisms due to difference in pressure, which is in agree- 

 ment with our conclusions. 



The movement of a low area down the St. Lawrence and 

 into the Gulf should be regarded as a passage across the coast 

 line, although Dr. Klotz makes the statementf that such pas- 

 sage is not marked by microseisms. This statement is not in 

 agreement with our results at Cheltenham, which is peculiarly 

 well located geographically for the study of such phenomena. 

 Of 300 microseisms recorded here between September 1, 1906, 

 and June 30, 1911, all but 32, about 10 per cent, have been 

 definitely connected with some change of pressure occurring 

 over the coast line between Labrador and Texas. 



That a change of pressure over land areas alone, although of 

 considerable intensity, does not produce appreciable tremors is 

 borne out by the following observations ; in many cases intense 

 depressions have developed over the Mississippi valley and 

 over the Lakes and have moved northward and eastward en- 

 tirely unaccompanied by microseisms until they had approached 

 sufficiently near the ocean to cause a steep pressure gradient 

 over the coast. Another small group in which a low develops 

 over the Gulf or the lower Mississippi valley and moves rapidly 

 northeastward, passing out to sea over the middle Atlantic 



* Journal Terrestrial Magnetism, vol. xiii. pp. 1-20, March, 1908. 

 f Department of the Interior, Canada, Eeport of Chief Astronomer, 1908, 

 pp. 24-40. 



