ON SEISMOLOGICAL INVESTIGATION. 



85 



of 



That low velocities are found on wave paths corresponding to chords 

 less than 10° suggests that this crust is not more than forty miles 

 in thickness. This seismometrical determination of thickness for the 

 earth's crust accords, it will be observed, with determinations of the same 

 quantity which are chiefly dependent upon the effects of high temperatures 

 assumed to prevail at such a depth. At fusion temperatures liquefaction 



[Fig. 3. — Average "Velocities for Preliminary Tremors if propagated 



along Chords.] 



150° 



is a state for many substances which is promoted by pressure, whilst at 

 still higher temperatures Arrhenius points out that whatever the 

 pressure might be it seems probable that fluids would become gaseous, and 

 such gases would be dense, but slightly compressible and viscous. What 

 the velocity table (as it now stands) indicates is that a crust passes 

 rapidly into a nucleus which is exceedingly rigid and fairly homogeneous. 

 A specific gravity can be defined for this nucleus which will meet the 

 requirements of gravitational observations, and it seems likely that the 

 same may accord with the tests of the astronomer. 



Isomorphous SalplioniG Derivativss of Benzene. — Fourth Report of the 

 Committee, consisting of Professor H. A. Miers (Chairman), 

 Dr. H. E. Armstrong (Secretart/), Professor W. P. Wynne and 

 Professor W. J. Pope. (Drawn up hy the Secretarij.) 



The object the Committee have primarily in view is the crystallographic 

 study of the complete series of sulphochlorides and sulphobromides derived 

 from the isomeric dichloro-, dibromo- and chlorobromo-benzenes. 



The results obtained in the case of the para- and two of the three 

 series of meta-derivatives have been referred to in previous I'eports. It 

 may be pointed out that whereas no evidence was obtained that the 1 : 4 

 derivatives exist in polymorphic forms — the five compounds measured 



