288 EARTHQUAKES. 



necessary. If we regard the earth as being solid, the 

 question resolves itself into the inquiry as to whether a 

 column of rock, which is equal in length to the diameter 

 of the earth, or about 8,000 miles, can be elongated 200 

 feet without a fracture. This is equivalent to asking 

 whether a piece of rock one yard in length can be 

 stretched one seventy thousandth of a foot. Considering 

 that this is a quantity which is scarcely appreciable under 

 the most powerful of our microscopes, we must also 

 regard this as a question which it is hardly necessary to 

 enter into calculations about before giving it an answer. 

 To vary the method of treating such a question, may we 

 not ask what is the utmost limit to which it would be 

 possible to raise up or stretch the crust of the earth 

 without danger of a fracture? Thus, for instance, to 

 what extent might a column of rock be elongated without 

 danger of its being broken ? From what we know of the 

 tenacity of materials like brick and their moduli of elas- 

 ticity, it would seem possible to stretch a bar of rock 

 8,000 miles in length for approximately half a mile 

 before expecting it to break. As to whether there is a 

 wave, the height of which is equal to half this quantity, 

 running round our earth as successive portions of its 

 surface pass beneath the attracting influences of the sun 

 and moon, is a phenomenon which, if it exists, would 

 probably long ago have met with a practical demonstra- 

 tion. 



The deformation which a solid globe or spherical shell 

 would experience under the attractive influences of the 

 sun and moon has been investigated by Lame, Thomson, 

 Darwin, and other physicists and mathematicians. 



A conclusion that we are led to as one result of these 

 valuable investigations is, that if the interior of the earth 

 be fluid, and covered with a thin shell, then enormous 



