1877.] Mr. C. S. Tomes on Vascular Dentine. 



43 



Before the introduction of the absolute system of electro-magnetic 

 measure, there was no means of estimating the electromotive force so 

 brought into play by the flow of a river or ocean stream, and the magni- 

 tude of the effect was perhaps overestimated. 



A Table is given, showing the electromotive force in volts produced in 

 two or three cases. 



Difference of potential be- 

 tween two sides in volts. 



Gulf-stream at lat. 30° N., long. 60° "W 8-6 



Equatorial current, lat. 10° N., long. 40° W. .. 10*0 



Dover and Calais tidal current 3*0 



Thames at Waterloo Bridge *016 



This electromotive force without doubt generates a current transverse 

 to the direction of the flow ; but since the surrounding still water or the 

 river bed or channel is not a non-conductor, any attempt practically to 

 detect it by plates placed on either side of the stream is not likely to 

 succeed, since the current through the galvanometer is only a derived 

 portion of the current in the stream. 



A comparison of a chart of ocean-currents with one of the isogonic 

 lines does not seem to show any distortion of the lines of equal variation 

 where they cut across. If, now, electric currents of any great magni- 

 tude were generated in ocean-currents, such would undoubtedly be the 

 case. Though Faraday's failure to detect any magneto-electric current 

 in the Thames may have been partly due to his employment of polarizable 

 electrodes, still there is evidence enough to show that these currents, 

 though certainly existing and capable of being produced on a laboratory 

 scale, cannot be regarded as contributing in any sensible degree towards 

 affecting the form and distribution of the isogonic lines. Those who 

 have looked to this as a possible partial cause of the irregularity observed 

 have been led, no doubt, by the dimensions of the streams to exaggerate 

 the magneto-electric induction caused by their flow. 



II. " On the Structure and Development of Vascular Dentine." 

 By Charles S. Tomes, M.A. Communicated by J ohn Tomes, 

 F.R.S. Received February 6, 1877, 



(Abstract.) 



The nomenclature and classification of the varieties of dentine have 

 hitherto been based solely upon the appearances discoverable in dried 

 teeth; in the present communication the author seeks to amend and 

 place upon a more satisfactory basis the grouping of these several kinds 

 of dentine, by bringing to bear upon their arrangement observations 

 upon the nature of the contents of those large tubes which give to the 

 tissues their name of " vascular " dentine, and, more especially, observa- 

 tions upon the methods by which they are developed. 



