THE EABTHQUAKE PROBKEM HECK 363 



wise unexplored medium through which the waves pass — the interior 

 of the earth. 



Tlie working tool of the seismologist is the seismograph. The 

 present seismographs, several types of which will be described, have 

 as their purpose as accurate recording as possible of the earth 

 motions during an earthquake. They have been developed from 

 the simple pendulum and many of them employ the principle of the 

 horizontal jDendulum. In this the weight or mass which corre- 

 sponds to the pendulum bob is held at a distance from a vertical 

 support by means of a boom which rests against a point near the 

 bottom of the support through a pivot to reduce friction. A diag- 

 onal wire from the weight to a point near the top of the support 

 carries the weight. The system is then a triangle free to swing 

 with one side as a nearly vertical axis (pi. 1, fig. 2). In an 

 actual earthquake the earth and the vertical support move and 

 through inertia the mass remains momentarily at rest. It is known 

 as a steady mass. If the earthquake continues the mass will take 

 up motion and provision must be made for this. In any case the 

 record or seismogram is a result of the different motions of the 

 earth and of the steady mass. The developments of the past 50 

 years have been directed toward securing the best solution of the 

 problem and toward refinements which give the desired accuracy for 

 recording distant earthquakes. 



Earthquakes occur in a 3-dimensional medium and waves approach 

 the seismometer from different directions according to relation of 

 position of earthquake to that of instrument. In the case of the 

 horizontal pendulum the record will be quite different if the axis 

 of the boom is directed toward or away from the earthquake or at 

 right angles to this line. Obviously the instrument must have a 

 permanent position, so the plan has been adopted of having two 

 instruments at right angles to each other and also, where possible, 

 a third instrument to respond to vertical waves. Formerly the hori- 

 zontal instruments were placed in north-south and east-west direc- 

 tions. The practice is now being developed of having the axis of 

 one boom point in the direction from which the greatest number of 

 earthquakes may be expected and the other is set at right angles to 

 this. For convenience we speak of the three components of a seismo- 

 graph, though each is a complete instrument in itself. Even though 

 the practice of directing the axis of one component as stated helps 

 to simplify the record, the results are not entirely satisfactory and 

 it is a very difficult problem to analyze completely the records of an 

 earthquake arriving from some other direction than those of the 

 two axes. 



