110 Classification and Nomenclature of the 



every instance. The change shewn by these experiments to 

 be necessary would increase considerably the mathematical 

 difficulties of the subject ; and they would be greater still, if 

 the change of bulk and lateral dimensions in the bodies 

 strained were included, according to the conclusions of Pois- 

 son, or the experiments of Wertheim, which are at variance 

 with each other. But these changes are so small in the 

 bodies I am contemplating, that they may be neglected 

 for all practical purposes. Thus, from my experiments, the 

 utmost extension of a bar of cast-iron, 50 feet long, is about 

 1 inch, or coo^h of the length, and therefore the change of 

 lateral dimensions of the bar being only a fraction of this 

 ¥ £oth, according to either Poisson or Wertheim, it is too 

 small to need including. The experiments in which I de- 

 duced the utmost extension of cast-iron, are given in the 

 " Report of the Commissioners on the Strength of Iron for 

 Railway Purposes." If the body strained were wrought-iron, 

 brass, or others of a very ductile nature, the change of late- 

 ral dimensions might, in extreme cases, be included. I 

 beg to mention, with great deference, that the profound work 

 of Lame, lately published, on " The Mathematical Theory of 

 Elasticity," in which the elasticity is considered as perfect 

 only, does not appear to apply to such bodies as I have here 

 treated of. — (Athenamm, No. 1353.) 



The Classification and Nomenclature of the Palaeozoic 

 Hocks of Great Britain. By Professor Sedgwick.* 



The Professor stated that the fossiliferous rocks formed in 

 reality only one great system, representing the whole succes- 

 sion of events from the first appearance of organic life to the 

 present day. But as it was convenient to divide history into 

 chapters, so the strata had been divided into three principal 

 series, — the Palaeozoic or Primary, the Secondary, and the 

 Tertiary, each characterized by many families, genera, and 

 species of peculiar fossils. The Palaeozoic strata might be 

 again divided into an upper, middle, and lower series : the 



* Read before the Meeting of the British Association for the Advancement 

 of Science at I full. 



