RIGIDITY OF THE EARTH 



99 



The observations are divided into two classes, the latter being 

 considerably more rehable than the former. 



TABLE I 



While the numbers in the second column agree in showing that 

 the earth's rigidity is of the order of that of steel, the differences 

 are so considerable that it is hardly likely that they can be relied 

 upon to within 20 per cent. 



If accurate, the third column would give a measure of the plastic 

 yielding of the body of the earth to the action of the distorting 

 forces of the sun and moon. As mentioned before, these should 

 all be negative, whereas the great preponderance is in the direction 

 of positive lag, which is meaningless; so that further than showing 

 that the lag is small (and therefore the viscosity high) these results 

 are practically valueless. 



It will be conceded that there is great need for more accurate 

 determinations, if our knowledge of the properties of the matter 

 constituting the earth's interior is to be increased; and it was in 

 the hope of obtaining results of a higher order of accuracy, as well 

 as such directness and simplicity of apparatus as practically to 

 eliminate all the difhculties and uncertainties which seem to be 

 unavoidable in the use of the horizontal pendulum, that the experi- 

 ments recorded here were undertaken. 



The prime object of the investigation is the determination of 

 the direction of the gravitational vertical, or rather of the changes 



