96 H. A. WILSON 
waves therefore travelled along nearly circular paths and may have been re- 
flected from the ground surface as shown in Fig. 20. In this way the arrival of 
several waves at different times may be explained. Fig. 21 shows a seismo- 
gram of the waves from 1000 pounds of dynamite at 9590 meters. In this case 
E S 
Fig. 20. 
y t 
fo) 
PO Nae 
o 
Fig. 21. 
there was a salt dome in the path of the waves. The curve for x the ground 
motion is also shown. The ground motion can be regarded as mainly due to 
several rapidly damped oscillations which, however, are not as well separated 
as in the previous case. 
An integraph? has been constructed which enables the curves for a; and az 
to be drawn quickly. Fig. 22 is a diagram of this integraph. FF’ is a brass 
block which rolls on two wheels WW’ and carries a pencil at its middle point 
x. The points FF’ on this block are connected to two parallel bars AD and 
A’D’ by means of two Peaucellier linkages which allow F and F’ to move 
along straight lines perpendicular to AD and A’D’ respectively. The bars 

Fig. 22. 
AD and A'D’ are connected by two links 4A’ and BB’ which are supported 
on pivots at S and T fixed to a base. The link AA’ can be turned about S by 
means of a liver SL. This lever also moves a bar ROP which can slide on the 
2 Intergraphs based on cne same g====al mrinciple are described in Les Intergraphs. Ab- 
dank-Abakauowiz. Paris, (1886). 
240 
