On the Matter composing the Interior of the Earth. 263 



vations for drawing an average, as there is no doubt that 

 spiral shells are not absolutely geometrically constant, and all 

 measures are therefore more or less approximate. 



One example of their use will suffice: this maybe Phasianella 

 striata. In this we have co = 16°; i by observation is 79°. 

 Also K = l*2; to this value corresponds in the Table 86° 41/. 

 Substituting these values of co and a in (1) we obtain i = 78° 41/. 

 Again, substituting in (4) for R and o>, we obtain ^=91° 36'; 

 by observation the same angle is 92°. These results are suffi- 

 ciently close to prove the regularity of the shell formation in 

 this case. 



It has not been thought worth while to study the effects of 

 compression on turbinated shells, because from their shape no 

 assumption can be made as to the direction of pressure that 

 could be considered generally applicable. 



XXXIII. On the Limits of Hypotheses regarding the Properties 

 of the Matter composing the Interior of the Earth. By Henry 

 Hennessy, F.R.S., Professor of Applied Mathematics in the 

 Royal College of Science for Ireland*, 



1. "TT^ItOM direct observation we are able to obtain only a 

 J- very moderate knowledge of the materials existing 

 below the solid crust of the earth. The depth to which we can 

 penetrate by mining and boring operations into this crust is 

 comparatively insignificant ; and these operations give us little 

 knowledge of the earth's interior in comparison with what is 

 afforded by the outpourings of volcanos. Two hundred active 

 volcanos are said to still exist, while geologists have established 

 thafc many thousands of such deep apertures in the earth's crust 

 have existed during remote epochs of its physical history. 

 The source or sources of supply for all these volcanoes have 

 poured out a predominating mass of matter in a state of liquidity 

 from fusion. Evidence is thus furnished that matter in a state 

 of fluidity exists very widely distributed through the earth. 

 The supposition that this fluid fills the whole interior, and that 

 the solid crust is a mere exterior envelope, is usually designated 

 as the hypothesis of internal fluidity. From this hypothesis 

 mechanical and physical results of primary importance in ter- 

 restrial physics may be deduced. 



Newton, Clairaut, Laplace, Airy, and other illustrious ma- 

 thematicians have used an extension of this hypothesis in dis- 

 cussing the earth's figure. They supposed the particles com- 



* Communicated by the Author, having been read before the Mathe- 

 matical and Physical Section of the British Association for the Advance- 

 ment of Science, Dublin, August 1878. 



