252 



SCIENCE 



comes warmed to steam temperature; but as 

 the mercury is being pushed steadily downward 

 where the tube is still cold, the oscillations 

 continue. Inasmuch as the center of gravity 



[N. S. Vol. XXXII. No. 816 



FiQ. 2 



of the mercury is not permanently changed 

 during one oscillation, the only work done is 

 that against friction. Energy must be sup- 

 plied to maintain this motion. 



The explanation given here is the one ad- 

 vanced by Griffiths' to explain a similar phe- 

 nomenon where a bulb of air was connected 

 to one end of a U-tube partly filled with mer- 

 cury. On heating the bulb with a gas flame 

 the mercury oscillated. Griffiths pointed out 

 that it would be useless to compare the quan- 

 tities of heat received and rejected, because 

 the expansive substance is constantly in con- 

 tact with conducting bodies at different tem- 

 peratures. He stated also that the action of 

 hot-air engines is of the same nature as the 

 one described; a prediction that was verified 

 by Webster" who, by means of an ingenious 

 device on a small hot-air engine, projected its 

 indicator diagram on a screen in a lecture 

 room. The diagram was of the form shown in 

 Fig. 2. 



P. E. Watson 



Univeesitt of Illinois, 

 May 19, 1910 



^ E. H. Griffiths, " The Thermal Measurement of 

 Energy," pp. 49-52, Cambridge Press, 1901. 



' A. G. Webster, " A Hot Air Engine Indicator 

 Diagram," Phys. Rev., 30, 264, No. 38. 



MICROSEISMS 



Ever since sensitive earthquake instruments 

 have been built, it has been found that outside 

 of earthquakes, which show characteristics in 

 their records peculiar to themselves, there, are 

 other disturbances recorded; disturbances that 

 manifest themselves by their continuity, ex- 

 tending over hours, days and even weeks. 

 They appear as small pulsations, the undis- 

 turbed trace of the seismogram being con- 

 verted to a finely serrated line. The ampli- 

 tudes may gradually increase to a millimeter 

 or even more on our instrument, and then dis- 

 appear again. In general they are far more 

 prevalent during the winter season than in 

 the summer. These pulsations or tremors I 

 call microseisms. The question naturally 

 arises, what produces these vibrations? Are 

 they due to a constant stress in the earth's 

 crust which at times adjusts itself by a rup- 

 ture along some weak line, along a fault, or 

 are they produced by thermometric or baro- 

 metric conditions of the atmosphere. Among 

 phenomena of the latter we may consider 

 winds, and the position and movements of the 

 area of low barometer. 



A very superficial examination of the facts 

 eliminates the temperature effect, that is, the 

 varying heat from day to day. 



For the barometric conditions of the at- 

 mosphere there was available at the observa- 

 tory the record of the Shaw microbarograph 

 and also the Canadian daily weather maps. 

 The microbarograph has a magnification of 

 20, so that rapid fluctuations in pressure are 

 very well shown, as shown by local strong 

 winds. The daily weather maps show the 

 position of the isobars from the Pacific to 

 Newfoundland in the Atlantic, with differ- 

 ences of pressure of 0.1 inch for adjoining 

 isobars. 



The seismograms are the records of two 

 Bosch photographic horizontal pendulums 

 mounted N.-S., E.-W., respectively, on a con- 

 crete pier free from the cement floor in the 

 basement of the observatory. The theoretical 

 magnification of the record is 120 and the 

 pendulums have air-damping. The periods of 

 the pendulum are adjustable and lie generally 

 between 6 and 10 seconds. The time-scale on 



