228 PROFESSOR FORBES ON A SEISMOMETER FOR MEASURING EARTHQUAKE SHOCKS. 



The following table may give a sufficient approximation : 



From the preceding table the values of *] . T and j 0' . T will be found by 



i / j 



looking in the third column for the logarithm of ~^ , or of four times the ratio of 



the squares of the extreme oscillations of two pendulums, derived from observa- 

 tion. 



Since V and v $' are known by the time of oscillation of the pendulums, 

 the value of T may be found, or the duration of the shock. 



The velocity of its motion will then be obtained from that of s, by Eq. (17). 



I am not aware of the methods which have been employed for estimating the 

 lateral shocks on railways (see Mr NICHOLAS WOOD'S Report on the Great Western 

 Railway), but I apprehend that some modification of the preceding instrument 

 might be found useful for such experiments. 



EDINBURGH, 27<A February 1841. 



