360 THE POPULAR SCIENCE MONTHLY. 



to give a rough sketch of the present state of knowledge in this com- 

 plicated subject. 



Although history abounds with more or less complete accounts of 

 earthquakes, it is remarkable that hardly ten years have yet elapsed 

 since an accurate record was first obtained of what actually occurs 

 during an earthquake. The combination of circumstances is curious, 

 by which a knot of Scotch students, working in Japan, has secured so 

 considerable an advance in seismology. The incessant but usually 

 non-destructive earthquakes by which Japan is visited, the strange 

 Japanese renaissance, and the importation of foreign professors, thor- 

 oughly trained at the Scotch universities in an accurate perception of 

 mechanical principles, are the three factors which have co-operated to 

 bring about this result. 



The Scoto- Japanese professors, of whom the most eminent are 

 Ewing, Gray, and Milne, have studied their subject with admirable 

 persistence, and have by their ingenuity placed seismologists in pos- 

 session of instruments by which the motion of the ground during an 

 earthquake is recorded on an accurate scale of time. Such instruments 

 are called seismographs, or recording seismometers. During an earth- 

 quake the ground and all that is fixed to it move together, and at first 

 sight it seems impossible to get anything to stay still during the vibra- 

 tion. An exact description of a scientific instrument would be out of 

 place here, but a general notion of these seismographs may be easily 

 grasped. 



The horizontal pendulum of Zollner, and a suggestion of Chaplin 

 (also a professor in Japan), are the sources from which "the horizontal 

 pendulum seismograph" of Ewing originated. The principles accord- 

 ing to which it is constructed may be explained as follows : If we con- 

 sider an open door which can swing on its hinges, and imagine that a 

 sudden horizontal movement is given to the door-post, at right angles 

 to the position in which the door is hanging, then it is clear that the 

 outer edge of the door will begin to move with a sort of recoil in the 

 direction opposite to that of the movement imparted to the door-post. 

 Since the door-post moves in one direction, while the edge of the door 

 recoils, somewhere in the door there is a vertical line which remains 

 still. The exact position of this line depends on the proportion which 

 the amount of the recoil of the outer edge bears to the direct motion 

 of the door-post. Now, if the sudden movement is imparted to the 

 door-post by means of the floor to which it is attached, it is clear that 

 a pencil attached to the door at this vertical line will write on the floor 

 the displacement of the door-post, notwithstanding that the floor has 

 moved. If next we suppose that there are two such doors hanging at 

 right angles to one another from the same door-post, and that a sudden 

 horizontal movement in any direction is given to the floor, each pencil 

 will write on the floor that part of the movement which was at right 

 angles to its door. Lastly, if the floor or surface on which the record 



