202 ROYAL SOCIETY OF CANADA 



the greater part of wliich are less than 150 fathoms deep. The distance 

 from Ottawa to the Gulf is about 700 miles, direction east-north-east; 

 and from Ottawa to the nearest broad waters of the Atlantic, off the 

 State of Maine, 300 miles, direction east-south-east. 



î^ext to the presence of a " Low " in the Gulf coincident with micro- 

 seisms we find the isobars to cut the valley of the St Lawrence (in which 

 lies the great fault) at right angles, that is, the gradient is along the 

 St. Lawrence valley, which is in general parallel to the Atlantic coast, 

 and to the line of the Alleghany mountains. 



Fu,rthermore, it is found that if a " High " prevails along the 

 south Atlantic coast, northward from Florida the microseisms are in- 

 tensified. 



'The passing of " Highs " and " Lows " across the coast-line, i.e., 

 from land to water, is not found to be marked by the occurrence of 

 microseisms. As the whole atmospheric movement is for Canada and the 

 United States from west to east, it is uncommon for a " High " or 

 " Low " to cross the coast-line from the Atlantic to the continent. 



It appears that the reversal of the position of " Low " and " High " 

 with reference to the Gulf for the former is not so closely associated 

 with the subsequent appearance of microseisms as obtains in the case 

 first stated. 



When there is a persistence of " l^ow " in the Gulf and " High " 

 on the Atlantic coast to the south as indicated, the microseisms set up in 

 the first instance become intensified in amplitude, so that the maximum 

 microseisms are not necessarily coincident Avith the greatest difference 

 of pressure, or the steepest gradients. It appears that the difference of 

 barometric pressure is in the first instance responsible for the micro- 

 seisms, and when favourable conditions continue the microseisms will 

 increase in amplitude, although the pressure difference may have de- 

 creased. Furthermore, another condition is that the line of " High " — 

 " Low " preserve its direction along the St. Lawrence valley. 



When a " Low " with even very steep gradients is to the west, say 

 over the Lakes, and " High " on the lower St. Lawrence or Gulf, micro- 

 seisms are generally weak or even absent altogether, although there are 

 exceptions. This is not the case when the " Low " is to the east, espe- 

 cially when over the Gulf. When the " Low " with steep gradients 

 moves up to Lake Erie by 8 A. M. of the day of the seismogram we may 

 fexpect to see the beginning of marked microseisms, which increase as 

 the " Low " moves down the St. Lawrence towards the Gulf. From the 

 immediately preceding it is seen that the microseisms give no indica- 

 tion of the approach of a "Low" or storm centre, but on the contrary 



