August 8, 1919] 



SCIENCE 



141 



these are found to be coincident with geologic 

 change. 



The lecture might well have been called 

 " The Philosophy of Paleontology," though 

 the evidences are drawn chiefly from the 

 vertebrates, the discussion being illumined by 

 a very interesting diagram of the pulse of 

 life, showing the influences of climate, con- 

 tinental elevation, and extinctions on the pul- 

 sations of vertebrate life. The discussion 

 follows such interesting topics as " Emergence 

 of Terrestrial Vertebrates," " Evolution of 

 Terrestrial Eoot," " Origin of Eeptiles " and 

 closes with the interesting comparison of the 

 graph produced by a sphygmograph, recording 

 the movements of the human pulse, with the 

 graph deduced from the study of geologic and 

 paleontologie evidences, recording the pulsa- 

 tions of life through many millions of years. 



In the closing lecture of the series Pro- 

 fessor Himtington discusses " Climate and the 

 Evolution of Civilization." This is a proper 

 closing for such a series, thus bringing out 

 the influence of physical factors in the high- 

 est form of evolution. The lecture discusses 

 the influence of climatic influence on certain 

 primitive tribes and nations of America and 

 is illustrated by a number of climographs. 



The volume is thus a discussion, in brief 

 form, of the chief factors bearing on the evo- 

 lution of the earth and its inhabitants from 

 the cosmieal origin to the culmination of the 

 highest phylum in the production of a high 

 type of civilization. Roy L. Moodie 



College op Medicine, 

 University op Illinois, 

 Chicago 



SPECIAL ARTICLES 



A PRACTICAL LONG-PERIOD SEISMOGRAPH 



A SEISMOGRAPH which is to record earth- 

 motion with fidelity must have a " steady 

 point" which shall be uninfluenced, as nearly 

 as may be, by that motion. In existing in- 

 struments this steady point is the center of 

 oscillation of a mass so suspended that when 

 disturbed it vibrates slowly about an equili- 

 brium position. The relative motion of the 

 ground with reference to this steady point is 



then recorded with magnification on a sensi- 

 tive surface by means of a suitable optical or 

 mechanical system. 



With suitable damping, and with a fault- 

 less recording system, the resulting record has 

 considerable accuracy for rapid seismic mo- 

 tion whose period is not greater than the free 

 period of the instrument; for slower motion 

 the accuracy declines rapidly; and the instru- 

 ment is wholly insensitive to motion whose 

 period is several times greater. It is there- 

 fore of prime importance that the period of 

 the instrument shall be as great as possible. 



When this period is made large, however, a 

 difiiculty arises in that the equilibrium posi- 

 tion of the heavy mass changes slowly, due to 

 tilts of the supix)rt in case of the horizontal 

 pendulum, to temperature changes in the ver- 

 tical-motion instrument, and in both due to 

 other less important causes. With the ordi- 

 nary method of recording, these slow wander- 

 ings are magnified in the record, increasing 

 the technical diificulty of obtaining the record, 

 and excessively increasing the size of sensitive 

 surface required. For these reasons practical 

 seismometry has limited itself to periods rarely 

 exceeding twenty seconds.^ 



Galitzin was the first to employ a method 

 which permitted the recording of seismic mo- 

 tion without registering the wanderings. He 

 employed an electromagnetic system which de- 

 pended upon the velocity of the earth-motion 

 rather than upon the displacement, thus avoid- 

 ing the slow changes. In doing this, however, 

 he sacrificed the flatness of the magnification 

 curve of his instrument, so that his record not 

 only did not represent the seismic motion 

 directly, but did not permit its computation 

 except in the case where such motion was 

 simply harmonic — a case which does not occur 

 in practise. 



The writer has devised a method of record- 



1 From the literature one might infer that in 

 practise, there is a low upper limit to the period 

 of the horizontal pendulum. Walker seta this limit 

 for laboratory purposes at forty seconds. But 

 Omori has achieved much longer periods. The first 

 pendulum constructed by the writer in the College 

 of Hawaii laboratory had a period of three 

 minutes. 



