82 MODERN SEISMOLOGY 
This leads to (1-2+)=0°35 or h-k=0'15 and along 
with =3 h this gives 4=0°38 leading to a general rigidity 
about three times that of steel. This, however, neglects the 
influence of compressibility. 
As a whole the position with regard to earth tides as 
indicated by movements of seismograph zero is rather unsatis- 
factory. The doubt that may very reasonably be entertained 
as to purely instrumental sources of error in the observations, 
renders theoretical discussion somewhat futile until we know 
exactly what the facts are. 
It is gratifying to know that the International Seismo- 
logical Association has in view experiments with horizontal 
pendulums at different points of the Earth, which ought to 
throw much light on the phenomena. It would also be useful 
if other means of experimenting could be devised. A solid 
surface undergoes, as we have seen, tilting of amount 
oWe 
Zz oe 
This must result in an apparent change of position of any 
star. But the effect is so small that it is hardly likely to be 
detected by astronomical means. On the other hand a liquid 
surface undergoes tilting of amount 
a £0) a 
The crerential tilting ee a cn surface and a solid 
surface beneath it is 
= ae -h+khk) ous 
The suggestion I a to make is that such an arrange- 
ment would show interference fringes parallel to the line of no 
resultant tilt, and that the direction would thus change in the 
course of the day. It might be practicable in this way to 
study the operation of the variable tilt 
I OW, 
et —h+ ae 
