SEISMOMETKY. 19 



and partially filled with mercury. To magnify the 

 motion of the mercury, small floats of iron rest on its 

 surface. These are attached by means of threads to a 

 pulley provided with indices which move in front of a 

 scale of degrees. We thus read off the intensity of an 

 earthquake as so many degrees, which means so many 

 millimetres of washing up and down of mercury in a tube. 

 The direction of movement is determined by the azimuth 

 of the tube which gives the maximum indication, several 

 tubes being placed in different azimuths. 



This form of instrument appears to have been sug- 

 gested by Mallet, who gives an account of the same in 

 1846. Inasmuch as the rise and fall of the mercury in 

 such tubes depend on its depth and on the period of the 

 earthquake together with its duration, we see that although 

 the results obtained from a given instrument may give us 

 means of making approximate comparisons as to the rela- 

 tive intensity of various earthquakes, it is very far from 

 yielding any absolute measurement. 



Another method which has been employed to magnify 

 and register the motions of liquid in a vessel has been to 

 float upon its surface a raft or ship from which a tall mast 

 projected. By a slight motion of the raft, the top of the 

 mast vibrated through a considerable range. This motion 

 of the mast as to direction and extent was then recorded 

 by suitable contrivances attached to the top of the mast. 



A very simple form of liquid seismoscope consists of a 

 circular trough of wood with notches cut round its side. 

 This is filled with mercury to the level of the notches. 

 At the time of an earthquake the maximum quantity of 

 mercury runs over the notches in the direction of greatest 

 motion. This instrument, which has long been used in 

 Italy, is known as a Cacciatore, being named after its 

 inventor. It is a prominent feature in the collection 



