28 Tlie Treasures of Siluria. [Jan., 



pression is more than double the resistance to extension, or as 

 100-7 to 47-7, being in the ratio of near 2:1. Hence it follows 

 that the most economic form of a steel bar undergoing a transverse 

 strain would be a bar with double flanges, having the area of the 

 top flange about one-half that of the bottom. 



This conclusion is borne out by the results of experiments on 

 transverse strain, where Sj , the strain per square inch of the material 

 at the elastic limit, =60 = 6 x 6-83 tons = 40-98, or 41 tons 

 nearly ; but the mean breaking-strain per square inch by extension 

 = 47-7 tons, clearly indicates that the compressive resistance in 

 the former case was considerably in excess of the tensile resistance. 



It is important in every experiment on the strength of materials, 

 which enters so largely into coiistructive art, that we should be 

 thoroughly acquainted with the properties of the material of which 

 the structure is composed, and that its resistance in all the difierent 

 forms of strain should be clearly and distinctly ascertained. In the 

 foregoing experiments we have determined the resisting powers of 

 the different specimens to bending, tension, and compression; but 

 we have omitted that of torsion, or twisting, until we have an 

 opportunity of doing so ujDon the same identical bars. These I 

 hope to accomplish at some future period, and also to give some 

 further results upon an enlarged scale, calculated to confirm what 

 has already been done, and to ascertain some additional fiacts in 

 regard to the changes now in progress in the manufactm-e of Iron 

 and Steel. 



IV. THE TEEASUEES OF SILUEIA. 



Thesaurus Silnricus : The Flora a7id Fauna of the Silurian 

 Period. With Addenda (from recent acquisitions). By John 

 J. BiGSBY, M.D., F.G.S. London : Van Voorst. 1868. 4to, 

 pp. 268. 



The " Treasures of Siluria " consist of a vast assemblage of those 

 " Medals of Oreation " with which the late Dr. Mantell, years ago, 

 made the intelligent reader familiar. These medals are couis of 

 various denominations, each of which had an unnumbered circu- 

 lation. But the region over which they severally passed current, 

 and the relative value which belongs to them, are questions which, 

 amongst others, we shall discuss in this article. 



At the head of our review we have placed the title of Imt one 

 book, — not because it gives us every possible information about the 

 " Treasures " whose value we wish to estimate, but because it is a 

 synopsis of everything that has hitherto been pubhshed on the 

 subject, and of not a little that is even now in the hands of the 



