I A Prof. H. W. Dove on a New Photometer. 



of surface, and may be temporarily buoyed up by the vesicular 

 character it may have retained from the ebullition of the liquid 

 in some places ; or, at all events, it may be held up by the vis- 

 cidity of the liquid, until it has acquired some considerable 

 thickness sufficient to allow gravity to manifest its claim, and 

 sink the heavier solid below the lighter liquid. This process 

 must go on until the sunk portions of crust build up from the 

 bottom a sufficiently close-ribbed solid skeleton or frame, to 

 allow fresh incrustations to remain bridging across the now 

 small areas of lava pools or lakes. 



32. In the honey-combed solid and liquid mass thus formed, 

 there must be a continual tendency for the liquid, in consequence 

 of its less specific gravity, to work its way up ; whether by 

 masses of solid falling from the roofs of vesicles or tunnels, and 

 causing earthquake shocks, or by the roof breaking quite through 

 when very thin, so as to cause two such hollows to unite, or the 

 liquid of any of them to flow out freely over the outer surface 

 of the earth ; or by gradual subsidence of the solid, owing to the 

 thermo-dynamic melting which portions of it, under intense 

 stress, must experience, according to views recently published 

 by my brother, Professor James Thomson*. The results which 

 must follow from this tendency seem sufficiently great and various 

 to account for all that we see at present, and all that we learn 

 from geological investigation, of earthquakes, of upheavals and 

 subsidences of solid, and of eruptions of melted rock. 



33. These conclusions, drawn solely from a consideration of 

 the necessary order of cooling and consolidation, according to 

 BischoFs result as to the relative specific gravities of solid and 

 of melted rock, are in perfect accordance with what I have 

 recently demonstrated f regarding the present condition of the 

 earth's interior, — that it is not, as commonly supposed, all 

 liquid within a thin solid crust of from 30 to 100 miles thick, 

 but that it is on the whole more rigid certainly than a continuous 

 solid globe of glass of the same diameter, and probably than one 

 of steel. 



II. On a New Photometer. By H. W. Dove J. 



BY means of our present photometric arrangements, the inten- 

 sity of two sources of light may, under certain circum- 

 stances, be measured; but it may be urged against them that 

 they are quite inefficient when the sources of light to be com- 



* Phil. Mag. S. 4. vol. xxiv, p. 395, " On Crystallization and Liquefac- 

 tion as influenced by Stresses tending to Change of Form in Crystals." 



t In a paper "On the Rigidity of the Earth," Proc. Roy. Soc. vol. xii. 

 p. 103. 



% Translated from the Monatsbericht of the Berlin Academy of Sciences, 

 May 1861. 



