198 liOYAL SOCIETY OF CANADA 



ally refers to the gradient between a " High " and a " Low " passing 

 through Ottawa. An examination has been made during the year of the 

 daily seismograms and the daily Weather Maps, including the isobars 

 or position of " Highs " or " Lows " and the forecast for the Ottawa 

 regions with reference to winds and storms. This examination furnishes 

 the data upon which the conclusions depend. As the Observatory is not 

 yet supplied with an anemometer and pressure gauge for comparison of 

 the dynamical conditions, we are at present dependent upon the daily 

 forecast. 



It may be stated at the outset, before discussing the preceding data, 

 that there is never a day in the year on which some trace of microseisms 

 can not be seen on a seismogram from a Bosch photographic seismograph. 

 It is all a matter of degree. I'hat microseisms should be ever present 

 is but natural, for the earth is in a continual state of stress and strain, 

 many varied and different causes contributing thereto. The term micro- 

 seisms as here used excludes any deviations of the vertical or movements 

 of the zero position of the pendulum. Some writers have divided 

 nxieroseisms into "earth tremors" or "pulsations," and "earth pulsa- 

 tions " or "pulsatory oscillations." The writer, however, from the 

 seismograms at this station sees no reason for this division, as it is not 

 at all evident from them that the contributory causes, whatever they 

 may be, manifest themselves in such a manner as clearly to differentiate 

 themselves. Furthermore, from the examination of the seismograms 

 the oscillations of the pendulum are excluded; on the one hand, from 

 the frequent change of period on the same seismogram, which would be 

 inadimissible for a pendulum, and on the other hand, if the pendulum 

 were made to oscillate we should expect to see the damping effect in 

 the decrease of amplitude, and a more or less sudden beginning, unless 

 the oscillations of the earth particles themselves were of a period com- 

 mensurable with that of the pendulum, which, of course, is sometimes the 

 case. It is evident that a photographic registering apparatus with high 

 magnification wall record microseisms when a seismograph with mechan- 

 ical registration will draw only a straight line. 



Of the e(Hitributing causes to stresses and strains and manifesting 

 themselves as microseisms, we may consider: — secular cooling of the 

 earth; unequal heating and the radiation during the day and night; 

 statical effect of atmospheric pressure, areal or local; dynamical effect 

 of atmospheric pressure, areal or local; precipitation, as rain or snow. 



The vanishing^ small effect of secular cooling, whatever its con- 

 stants may be becomes evident from the fact, that, although it is ever 

 present, and its manifestations w^ould be of a constant nature the re- 



