N 



^ 



u 



Med 



Mi 



and 



's, the 

 aentlj 





», 



wore 



) 



liave 



d 



* 



itions 

 (have 

 m tlie 



_reis 



iiiued 

 J- are 

 [or to 



^ bj 

 •ater. 



•nto 



tliat 



rres- 

 l.for 



iit7- 



Iri^ 



b 



Ch. XXXIL] 



THICKNESS OF EAETH'S CEUST. 



203 



according to the rate of compressibilitj deduced from experi- 

 meiit^ would have its density doubled at the depth of 93 miles^ 



miles 



Dr. 



com 



compressed into one-fourth^ and stone into one-eighth of its 

 bulk."^ It is more than probable^ however, that after a 

 certain degree of condensation, the compressibility of bodies 

 may be governed by laws altogether different from those 

 which we can put to the test of experiment ; but the limit is 

 still undetermined, and the subject is involved in such ob- 

 scurity, that we cannot wonder at the variety of notions 

 which have been entertained respecting the nature and con- 

 ditions of the central nucleus. Some have conceived it to be 

 fluid, others solid; some have imagined it to have a cavernous 

 structure, and have even endeavoured to confirm this opinion 

 by appealing to observed irregularities in the vibrations of 

 the pendulum in certain countries. 



An attempt has been made by Mr. Hopkins to determine 

 the least thickness which can be assigned to the solid crust 

 of the globe, if we assume the whole to have been once 

 perfectly fluid, and a certain portion of the exterior to have 

 acquired solidity by gradual refrigeration. This result he 

 has endeavoured to obtain by a new solution of the delicate 



motion 



mentioned 



of the sun 



moon 



moon, on the 



protuberant parts at the earth's equator ; for if these parts 



motion 



om 



perfectly fluid, and incrusted over with a thin shell only a 



miles 



In other words, the disturbing action of 



same 



same 



mass 



globe in which the solid shell forms one-half of the 

 and another in which it forms only one-tenth. 



Mr. Hopkins has, therefore, calculated the amount of 

 precessional motion which would result if we assume the 



^Young's Lectures, and Mrs. Somerville's Coiiuection of the Physical Sciences, 

 p. 90. *^ ■ 



