318 



EARTHQUAKES. 



wire. This iiistrumeat is shown 

 drawing. 



in the accompanying 



Another apparatus is 

 the Microseismograph of 

 Professor Rossi. Here we 

 have an arran gement which 

 gives automatic records of 

 sh'ght motions. It consists 

 of four pendulums, each, 

 about three feet long, sus- 

 pended so that they form 

 the corners of a square 

 platform. In the centre 

 of this platform a fifth, but 

 rather longer, pendulum is 

 suspended. The four pen- 

 dulums are each connected 

 just above their bobs to 

 the central pendulum with 

 loose silk threads. Fixed 

 to the centre of each of 



these threads, and held 

 Fig. 37. — Normal Tromometer, x- ^^ u t i,^ 



B, bob Of pendulum ; P, prism ; m, microscope ; Vertically by a bght Sprmg, 



s, rim of scale. jg ^ needle, so adjusted 



that each thread is depressed to form an obtuse angle 

 of about 155°. These needles form the terminals of an 

 electric circuit, the other termination of which is a small 

 cup of mercury placed just below the lower end of the 

 needle. By a horizontal swing of one of the pendulums 

 this arrangement causes the needle to move vertically, but 

 with a slightly multiplied amplitude. By this motion 

 the needle comes in contact with the mercury, and an 

 electro magnet with a lever and pencil is caused to make 

 a mark on a band of paper moved by clockwork. The 



