372 



Dr. J. J. Bigsby's Brief Account of the [Feb. 21 



sistance of 5^ inches of the fine platinum wire in the cross wire, 5 inches in 

 the armature-branch, and 4 feet in the electro-magnet branch. 



When there was no extra resistance in either of the branches, the length 

 of the cross wire being only about a few feet, the intensity of the current 

 in the electro-magnet branch, compared with that in the cross wire, was as 

 1 : 60 ; and when the resistance of the primary coil of the inductorium was 

 interposed in the cross wire, the relative intensities were as 1 : 42. 



In conclusion I will mention that there is an evident analogy between 

 the augmentation of the power of a weak magnet by means of an inductive 

 action produced by itself, and that accumulation of power shown in the 

 static electric machines of Holtz and others which have recently excited 

 considerable attention, in which a very small quantity of electricity directly 

 excited is, by a series of inductive actions, augmented so as to equal, and 

 even exceed, the effects of the most powerful machines of the ordinary con- 

 struction. 



February 21, 1867. 

 Dr. W. A. MILLER, Treasurer and Vice-President, in the Chair. 

 The following communication was read : — 



" A brief Account of the ' Thesaurus Siluricus,^ with a few facts and 

 inferences." By J. J. Bigsby, M.D. Communicated by Sir 

 R. I. MuRCHisoN, Bart. Received January 28, 1867. 



I have been led to attempt the preparation of a general view of Silurian 

 life, as far as now known, by my own frequent want of such a record or 

 muster-roll of the constituent members of this great initiatory division of 

 palaeozoic zoology, — a task which has been made pleasant by some personal 

 knowledge of two countries rich in the earlier formations. 



I have been further encouraged by the great accumulations of the last 

 few years, through the establishment in North America and elsewhere of 

 numerous colleges, each of them having become the centre of more or less 

 field-work. Far more aid still has been derived from many public surveys 

 on a tolerably liberal scale. Nor can we forget the highly meritorious and 

 successful labours which have been, and still are, carried on by private 

 individuals in almost every part of Europe and North America. 



As this undertaking required an exactitude and a critical skill in deter- 

 mining species and genera according to late improvements in classification, 

 much beyond an ordinary acquaintance with Silurian life, after my mate- 

 rials were put together, I obtained the very valuable aid of Mr. J. W. 

 Salter, late Palaeontologist at the London Museum of Practical Geology. 



I was then, through the kindness of Sir Roderick I. Murchison, Bart., 

 allowed to submit my manuscript to Robert Ethcridge, Esq., F.R.S.E., 



