472 J. E. BurbanTc — One Phase of Microseismic Motion. 



states, — in such cases no appreciable microseisms occur until it 

 approaches the coast, when they begin and reach their greatest 

 intensity while the center is passing out to sea. Another very 

 rare condition is when a low develops over the Gulf states and 

 moves northeastward along the Allegheny mountains, passing 

 into Canada without producing any great pressure changes 

 along the coast line ; in such cases no microseisms are recogni- 

 zable. Still another very rare case is when a low develops 

 over the ocean east of Florida and recurves northwestward, 

 passing inland over the South Atlantic coast. The micro- 

 seisms rapidly decrease after the center passes inland, although 

 it may still be of considerable intensity. 



Of the 268 microseisms recorded here during the last 5 years 

 and which appear connected with atmospheric variations, 

 approximately two-thirds occur in the period October to April, 

 when pressure changes are more frequent and abrupt ; they 

 occur very rarely during June, July, and August, when pres- 

 sure gradients are very small. During these winter months 

 these microseisms often continue for several days, diminishing 

 and increasing in intensity as a succession of abrupt pressure 

 changes from low to high sweep over the coast into the Atlan- 

 tic Ocean. 



A detailed study of all these cases confirms the general con- 

 clusions already set forth in connection with my earlier paper ; 

 hence the tabulation and detailed notes are omitted from this 

 paper, and only conclusions stated. 



Of the 268 microseisms above mentioned, 74 were connected 

 with lows moving over the Gulf of St. Lawrence ; 20 of these 

 were of sufficient amplitude to determine the period, which 

 varied from 2*8 to 3-5 seconds, with 4 cases of 3*6, 4*6, 5*0, 

 and 60 seconds respectively — 68 lows moved wholly or in part 

 over the coast of New England ; of these 21 showed periods 

 ranging from 3'0 to 3*5 seconds, with one 3'8, one 4*0, and two 

 5*0 seconds, the remainder being too ill-defined to allow deter- 

 mination of period — 73 microseisms were connected with pres- 

 sure changes occurring over the Middle Atlantic coast between 

 New York City and Cape Hatteras ; nearly all of these were 

 lows and show periods ranging from 3*0 to 3*5 seconds, with 5 

 cases ranging from 3*8 to 5*0 seconds. There were 20 cases 

 connected with the South Atlantic coast, nearly all being due 

 to lows passing northeastward into the ocean and often moving 

 northward parallel to the coast with decreasing intensity ; most 

 of these gave intense microseisms with the usual period, one 

 case having a period of 5*0 seconds ; in addition to these 

 were 13 cases of lows forming in the Gulf, or the ocean east 

 of Florida, also including hurricanes which approach the Flor- 

 ida peninsula or the Gulf coast ; these show the usual periods 



