Notices of Memoirs — Rev. E. Hill — Density of Meteorites. 517 



But in this enumeration is included the great Cranbourne meteorite, 

 which weighs 3|^ tons, and exceeds all the rest put together. A 

 mass so great swamps the rest, and makes the calculation unsatis- 

 factory, affording at best a maximum limit. Several others are very 

 large. Suppose we exclude all over 250 lbs. There remain fifty- 

 two cases, and the above process applied to these yields as result a 

 specific gravity of 4-68. 



I have tried yet another method. I have seen somewhere a state- 

 ment that in the British Museum are specimens of 260 meteorites, 

 of which 55 are metallic. Now it becomes obvious on tabulating 

 the specific gravities that the vast majority are either metallic with 

 specific gravities lying between 7 and 8, or stony with specific 

 gravities between 3 and 4. Separating the 57 cases previously 

 alluded to into these two groups, and finding the respective average 

 specific gravity by the second method, the results are 7'61 and 3*78. 

 From 55 specific gravities of 7-61 and 205 of 3-73, we get an average 

 of 4*55. This method is open to the objections made against the 

 first, but in a slightly less degree from the larger numbers dealt 

 with. 



It is probable that metallic masses more readily attract attention 

 than stony meteorites, but on the other hand some appear to decay 

 very rapidly. Thus it does not seem possible to say that either 

 class has a better chance of obtaining admission to our calculations ; 

 or a preponderating influence on the results. We may say, therefore, 

 as a general result, that an assemblage of such meteoric masses as 

 fall upon the earth, if collected from space indiscriminately and 

 aggregated into a single mass, would form a body whose specific 

 gravity would probably lie between 4-5 and 5*7, so long as its dimen- 

 sions were moderate. 



Such a body, however, if it grew to be of planetary size, might 

 become denser from the pressure due to its own attraction. How 

 much denser we cannot say. But since water has been compressed 

 by i\-th of its bulk under a pressure of 2000 atmospheres (Ency. 

 Brit., art. Elasticity), and a like pressure would be experienced at 

 a depth of about four miles of the earth's crust, at least as much 

 may be expected in such an aggregated mass. Such a compression 

 would raise even the lowest of the above results, viz. 4*5, to 4-9. 

 And twice the compression would raise it to 5-4. Now 5-6 is the 

 A'alue usually adopted as the best estimate of the specific gravity 

 of the earth. The density of the earth is, therefore, perfectly 

 consistent with its being an aggregation of meteoric materials. 

 Whether we regard meteorites as fragments of a planet, as 

 condensations of cometary matter, or as self-existent bodies, they 

 are certainly specimens of matter scattered widely over the solar 

 system, and may very possibly be fair samples of its materials. 

 And small as is the catalogue even of all we know, they contain all 

 the most important constituents of the earth's crust, and even of its 

 soil. 



If the earth be an aggregate of meteoric materials, so must be the 

 other planets. Now the densest of these is Mercury with a specific 



