364 POPULAR SCIENCE MONTHLY. 



of observation so widely separated corresponded. Japanese, Indian, 

 and American earthquakes could be simultaneously studied in Italy, 

 Russia, Germany, and England; and thus a new, hitherto undevel- 

 oped field was gained, the scope of which has already extended far 

 beyond its merely geological aspect. 



This could have happened only through another advance that has 

 been made in our century, which has first rendered a real seismology, 

 a scientific knowledge of the seismic conditions of the earth, possible 

 through the immense development of technics, by which a system of 

 instrumental observation of earthquakes was established. Only 

 through this could the acquisitions of recent times be utilized. 

 While formerly observations were macroscopic and touched only 

 earthquakes that could be directly felt, they now cover essentially 

 microscopic tremors of the earth's crust, of less than a thousandth of 

 a millimetre, that are wholly imperceptible to human senses; and we 

 can read them, enlarged at our pleasure, on our photographically 

 registering seismometers. We already had instruments which cor- 

 rectly indicated the time of the beginning and possibly the direction 

 of a shock; but we needed and have invented new instruments — 

 various sorts of horizontal and vertical pendulums — for the observa- 

 tion and representation of the whole course of the movement. The 

 vertical indicating instruments are much used in Italy, and the hori- 

 zontal ones almost exclusively in England, Japan, and Germany. 

 The horizontal pendulum was invented in Germany in 1832 by 

 Hengler, adapted to scientific use by Professor Zollner, of Leipsic, 

 and afterward applied in that form by English, German, and other 

 observers. The most complete shape and the one best adapted to 

 extremely delicate seismic observations was given to it by the late 

 German astronomer and geographer Dr. Ernst von Rebeur Pasch- 

 nitz, of Merseburg. Having undergone a few small changes, fixed 

 in a threefold combination it serves as our most sensitive and accurate 

 seismometer. Its movements and its very exact time markings are 

 photographically represented. The pendulum box is only forty cen- 

 timetres in diameter. In consequence of its convenience and cheap- 

 ness, its self-action and its serviceability, it is becoming adopted more 

 and more generally as an international instrument. 



Microseismic investigation and its wide extension over the earth 

 have raised seismology another step during the last twenty years, so 

 that it may be said that really exact seismic research began with 

 it. Modern seismology has confirmed many of the older results, such 

 as the localization of earthquakes on the shores of the Pacific, the 

 Mediterranean and in the mountain chains of the earth, and also the 

 importance of homoseists and the epicenter. It has, on the other hand, 

 greatly modified the former estimates of the velocity of propagation 



