EARTH TREMOES. 319 



five pendulums being of different lengths allows the 

 apparatus to respond ' to seismic waves of different velo- 

 cities.' ^ 



Lastly, we have Professor Rossi's Microphone, This 

 consists of a metallic swing arranged like the beam of a 

 balance. By means of a moveable weight at one end of 

 the beam this is so adjusted that it falls down until it 

 comes in contact with a metallic stop. This can be so 

 adjusted that a slight tap will cause the beam to slightly 

 jump from the stop. The beam and the stop form two 

 poles of an electric circuit, in which there is a telephone. 

 The slightest motion in a vertical direction causes a 

 fluctuation in the current passing between the stop 

 and the beam, and a consequent noise is heard in 

 the telephone. 



With instruments analogous to these, observations 

 have been made by various observers in all portions of 

 Italy, extending over a period of ten years. Every 

 precaution appears to have been taken to avoid accidental 

 disturbances, and the experiments have been repeated in 

 a variety of forms. 



Results obtained in Italy. — The results which from 

 time to time have been announced are of the greatest 

 interest to those who study the physics of the earth's 

 crust, and they appear to be leading to the establishment 

 of laws of scientific value. 



It would seem that the soil of Italy is in incessant 

 movement, there being periods of excessive activity 

 usually lasting about ten days. Such periods are called 

 seismic storms. These storms are separated by periods 

 of relative calm. These storms have their greater regu- 

 larity in winter, and sharp maximums are to be observed 

 in spring and autumn. In the midst of such a period or 

 ' Tel. Journ., November, 15, 1881. 

 15 



