STANDARDIZATION OF SEISMOGRAPHS 23 
ment and the observed deflection on the paper noted, and with 
his published data of amplitudes in millimetres he gives the 
angular tilt required to produce 1 mm. deflection. 
Now the fundamental equation 
6+ 260+ 7120 = -£¢/l+ g/l 
shows that for a steady tilt yw, we get 
¢, == 
and since the deflection on the paper say y, is L@, where L is 
the length of the boom we may calculate 7 by the formula 
pers 
wy, 
As an actual example we have 
i100 n= =0'349 
and the experiment gave a deflection of 1 mm. for 043 
tilt so that 7= 16:8 and L/7=6. 
L and Z are of course constants that may be determined 
once for all. Thus while we must admit that in a complicated 
record it would be practically impossible on account of the 
“free” terms to assign the true magnitude of the horizontal 
earth movement in absolute measure, there are certain cases 
(notably sharp impulses) in which the earth movement can be 
determined from the record. This point has not always been 
recognized with regard to the Milne Seismograph. 
In the Wiechert Seismograph artificial air damping is 
introduced. We shall first suppose that the friction introduced 
enables us to write the equation in the form 
M#?6 +26 4+ (uk? - Mgh)O= - Mht 
or 6+ 266+ 270= — </L 
When the expression 1/U which determines the magnifica- 
tion is plotted for different values of «and of the damping 
ratio v, it appears that the magnification remains more nearly 
constant from ~=0 to z=1 when v is about 5 than for other 
values of v, and this ratio is aimed at in practice. The corres- 
ponding value of e/z is about 0°45. This comparatively large 
damping ratio makes it difficult in practice to get a sufficient 
