[klotz] MICROSEISMS 199 



corded microseisms are of tlitc most fluctuating character both in time 

 and magnitude, complete!}^ masking the effect of secular cooling. The 

 daily alternations of unequal heating and radiation during the 24 hours 

 are not shown by their effect on microseisms. The case of precipitation 

 is similar in regard to microseisms to the preceding. It may be noted 

 that the stresses set up over large areas, hundreds of miles in extent, by 

 differential loading of rain, is small compared with that of barometric 

 pressure. Taking an area, say of a thousand miles with a rain- 

 fall of one inch which is a pretty heavy rain, and decreasingly 

 distributed, we would have a maximum pressure of a little over one- 

 thirtieth of a pound per square inch, and the rain-pressure diminishing 

 to zero for the edge of the area. An average barometric gradient, on the 

 otlier hand, over such an area would be several times as great, due to a 

 differential atmospheric pressure equivalent to about three-tenths of an 

 inch of the mercurial barometer. The rain-pressure may make itself, 

 however, felt locally as has been observed. The result of a heavy rain- 

 fall soon fills the valleys and streams much beyond the direct precipita- 

 tion on them, so that this loading and bending of the surface may become 

 a measurable quantity by an observing station in the neighbourhood. 

 This effect is, however, one of tilting, of change of vertical or change of 

 pendulum zero and not microseisms, the subject at the moment under 

 discussion. 



Tlie effect of difference of atmospheric pressure and of change of 

 atmospheric pressure may be manifested in two ways by the seismograph. 

 We are here dealing with large areas say 1,000 miles in extent, for 

 local barometric conditions have little or nothing in common with 

 microseisms. In the one case, considering the earth as having an elastic 

 crust, the pier is tilted towards the area of greatest pressure, jn con- 

 sequence of which the pendulum will move in that direction, i.e., its 

 zero line will be displaced. Besides this effect of statical loading, there 

 appears to be no doubt, based on the records here, that vibrations are set 

 up by this statical loading, quite apart from the dynamical effect of 

 change of pressure. In the other case, by change of pressure over a 

 wide area vibrations are set up on the earth's surface, and these may be 

 produced by two causes from the one plienomenon. The one of these is 

 the passage of " Highs " and " I^ows " over the surface, equivalent to 

 the dragging of a weighted meniscus over the surface; and the other is 

 the winds set up or resulting from the atmospheric gradient due to 

 difference of pressure. The action of the winds would most likely 

 operate through friction resistance along the surface of the earth ra'ther 

 than through impact on unevenness of surface or obstructions. In 



