﻿Certain Questions connected with Astronomical Physics. 571 



gravitational stress is insufficient to entail any such "welding " 

 together of cold ingredients ; it is only at some depth below 

 the surface [relatively insignificant, no doubt, compared with 

 the diameter o£ the earth] that such crushing effect could 

 take action. Some minor irregularities o£ surface would 

 thus be inferable from this condition (on our globe). 



It may be worth noticing that recently in ' Nature ' it 

 was remarked that earthquake-waves, the velocity of which 

 is now instrumentally measured, appear to indicate a rather 

 abrupt change* of quality or density in the globe's interior, 

 at a point about 30-40 miles from the surface. This con- 

 sideration becomes of practical value in relation to possible 

 additions to knowledge in connexion with earthquakes. Thirty 

 miles thickness of crust, compared with the globe's diameter, 

 is evidently but a thin shell. 



Some of the observed internal heat of the earth's crust might 

 be attributable to the chemical action of material of diverse 

 nature, without necessarily appealing to an original nebulous 

 condition. We must take all evidence and cooperative 

 causes into consideration, — every possible evolutionary process, 

 slow, as well as catastrophic. The observed tide-resisting 

 quality of our globe, whereby it has been inferred to resist 

 distortion like glass or steel, — has, at any rate, to be har- 

 moniously | accounted for. In 'Nature,' August 9, 1906, 

 the following expression of view by Sir George Darwin is 

 quoted : — 



u On the whole, we may fairly conclude, whilst there is 

 some evidence of a tidal yielding of the earth's mass, that 

 yielding is certainly small, and the effective rigidity is at least 

 as great as steel " (p. 358). 



Demonstrably, there is much iron present in our universe, 

 while this metal is known to be a chief constituent of meteorites 

 (from actual analyses). Looking to a purpose to be fulfilled 



* In an address at the recent meeting of the British Association by 

 Principal E. H. Griffiths, attention is directed to a conclusion of Prof. 

 Milne's, namely : — 



" The difference in the rate of propagation of earthquake-waves 

 through the earth's interior and through the crust, has led him [Prof. 

 Milne] to the conclusion that the material below a depth approximating 

 to thirty miles is of uniform nature, and that the change in physical 

 constitution is abrupt at some such depth as that indicated ? ' (' Nature,' 

 August 9, 1906, p. 358). 



t On the other hand, it is a merely speculative idea, that pressure 

 (extremely great) might at the temperature of fluidity solidify steel. It 

 may be worth trying a much greater pressure than is applied in the 

 instance of the " fluid compressed steel " of "YYhitworth, as a research 

 experiment. 



