The Causes and Phenomena of Earthquakes. 55 



and earthquakes are independent of any considerable influence 

 from external causes. 



This problem was endeavoured to be solved by Hopkins, who 

 assumed the structure of the earth under two or three supposed 

 conditions/ 



The earth niay have a solid or fluid centre, or it may have a 

 fluid ring interposed between a solid centre and a cooled surface. 

 Mathematically discussed, the following result is arrived at — that 

 the crust of the earth cannot have a less thickness than l-4th or 

 l-5th of the radius of its external surface, i. e., about 800 or 1000 

 miles, more or less. The question as to precession and nutation 

 is in relation to this result. 



Notwithstanding these views, there are considerations which 

 permit the opinion that the derangements in the interior of the 

 earth may, in part, be due to interference of the sun and moon. 

 There are, however, difficulties in the solution of Hopkins's pro- 

 blem in the third case which render it almost impossible. 



By a paragraph in an English journal which arrived in July, it 

 appears that observers had gone out to Vesuvius to ascertain 

 whether the views of Signor Palmieri are correct as to the in- 

 fluence of the moon on the eructations of that volcano. The 

 statements to that effect by Professor Perrey were submitted by 

 the Academy of Sciences at Paris, to a Commission which has 

 affirmed that for half a century earthquakes have been more 

 frequent at the new and full moon than at the quadratures. 



M. Zantedeschi's views go to the extent of assuming that there 

 is a terrestrial as well as oceanic tide produced by sun and moon. 

 But we have already heard the opinions of Herschel and Hopkins ; 

 and Mr. Mallet, who treats with caution the statements made, 

 admits that sun and moon do influence phenomena in the earth ; 

 but, certainly the cause of eruptions and earthquakes must be 

 sought within the earth. 



The facts themselves relating to the temperature belo «v the sur- 

 face are various, and the actual mean ratio of depth corresponding 

 with a degree actually undetermined ; and it is easier to accumu- 

 late examples than to account for the cause of the heat. Still, the 

 general assumption is generally considered in the main to be cor- 

 rect, that at a certain depth below there must lie a fluid zone, pro- 

 bably very irregular in its roof, so as to allow of cavernous places 

 above its ordinary level ; or a succession of reservoirs, either 

 insulated or connected, in which the fluid is collected, in the 

 solid crust. 



Whatever may have been the original condition of the earth, 

 it is impossible to deny the influence of pressure, and this, in Mr. 

 Hopkins's view, is opposed to a moderate thickness of the earth's 

 crust, such as Professor Bischoff admits, viz., one of twenty or 

 thirty miles. 



