170 Notices of Memoirs — D. Forbes, F.R.S., 



vertible evidence has been produced to prove that the temperature 

 of the earth increases in direct proportion to the depth ; so that it 

 seems most probable that the combined effects of expansion and 

 heat would more than counteract any tendency to solidification due 

 to the influence of pressure. 



Having now taken into consideration the various objections which 

 have at various times been urged against the theory of the earth's 

 internal fluidity, as well as devoted some consideration to the op- 

 posing view of its solidity, it will be noticed, if we pass in review 

 the more distinctive features of the two hypotheses, that the former 

 theory is a legitimate deduction from the data afforded by the direct 

 study of the earth itself, whereas the latter, on the contrary, instead 

 of making the explanation of the earth's phenomena its starting-point, 

 devotes itself all but exclusively to the task of proving that it could 

 not be fluid. Thus, how is it possible, if the earth's mass be solid 

 throughout, to account for the great upheavals and sinkings down of 

 large portions of the rock formations which compose its external 

 surface ; do not these phenomena lead to the direct inference that 

 the external crust cannot by any possibility rest in depth upon any 

 unyielding mass of matter in a solid condition, but that it must 

 necessarily be superposed upon some more or less fluid substance, 

 which by its mobility can, when some one portion of the crust 

 above it sinks down, become displaced, and so make room for it by 

 elevating or as it were floating up some other part of the same ? 



In like manner the hypothesis that the earth is essentially solid 

 necessitated that the phenomena of Volcanos should be explained 

 upon the supposition that they had their sources in numerous small 

 isolated basins of molten lava scattered over the surface of the globe, 

 a view which is totally inconsistent with the results of chemical and 

 mineralogical investigation, which proves that the ejected products 

 are identical in constitution, even if taken from volcanic vents the 

 most distant from one another ; nor does such a theory attempt in 

 any way to explain the tidal phenomena of volcanic outbursts and 

 earthquakes previously referred to. 



So far, therefore, as we haA^e gone into this subject, we may regard 

 the balance of evidence as indicating that at a depth of about fifty 

 miles, or less, from the surface, there exists a continuous zone of 

 molten rock or lava, such as is brought up to the surface in volcanic 

 eruptions. Let us now consider how deep this zone or stratum of 

 molten matter is likely to extend, and also what forms the more 

 central mass of the earth below it. 



In order to answer these questions, we must look for information 

 to other than direct evidence, and first of all may inquire as to 

 whether the consideration of the mean density, i.e., expressed in 

 other words, the actual weight of the entire earth itself, can throw 

 any light upon these abstruse points. The consideration of the at- 

 traction which bodies exert upon one another in the ratio of their 

 respective magnitudes has enabled the physicist to effect the at 

 first thought apparently impracticable task of weighing the entire 

 earth itself; it is, however, out of our province on the present oc- 



