to 250-fold the 

 degree of mag- 

 nification of the 

 actual movement 

 of the earth par- 

 ticles. The in- 

 strument is now 

 arranged to re- 

 cord a magnifica- 

 tion of about 

 150-fold, so that 

 if the recording 

 needle swings one 

 and one-half inch, 

 the actual move- 

 ment of an earth- 

 particle is one- 

 hundredth of an 

 inch, provided 

 that the direc- 

 tion of wave mo- 

 tion be either 

 north-south or 

 east-west. Waves 

 whose direction 

 lies between these 

 points of the com- 

 pass produce effects on the needles which can be calculated on the principle 

 of the parallelogram of forces in physics. Thus the direction from which 

 the waves come can be learned. 



A heavy quake produces waves which are felt or recorded all over the 

 earth, reaching the instnuuent in three ways. They ccnne through the 

 earth's mass by the shortest line, producing the first preliminary tremors, 

 or by reflected lines, producing the second preliminary tremors; through 

 the earth's crust by the shortest distance, the minor arc, producing the long 

 waves and the trailers; and through the earth's crust by the longest distance 

 or major arc. Sometimes the first preliminaries are not recorded on a 

 machine like that at the Museum because they come from so great a <listance 

 as to emerge in a nearly vertical direction and the machine is not affected 

 by such wave motion. It is not easy for the amateur to recognize the record 

 produced by the waves arriving by the major arc. 



The seismograph at the Museum, a large-model Mainka, manufactured 

 by J. and A. Bosch of Strassburg, Germany, has been deposited with the 

 institution by the New York Academy of Sciences, to which it was given 

 by Mr. Emerson McMillin. 



290 



The drum in the foreground rotates toward the observer, draw- 

 ing a band of smoked paper beneatli a recording needle. This 

 needle is pivoted to an arm running Ijack to a point directly over 

 the center, where it connects by a system of levers with the exact 

 center of the steady mass, to which it is rigidly fastened 



