Prof. J. W. Mallet on the Density of Solid Mercury. 145 



air, required over dilute sulphuric acid 3 min. 30 sec, over ether 2 

 min. 25 sec. A brass needle of the same dimensions exhibited 

 similar effects. 



In conclusion the magneto-electric induction taking place in 

 moving masses of water on the earth's surface under the influence 

 of terrestrial magnetism is briefly discussed. 



Before the introduction of the absolute system of electro- 

 magnetic measure, there was no means of estimating the electro- 

 motive force so brought into play by the flow of a river or ocean- 

 stream, and the magnitude of the effect was perhaps over- 

 estimated. 



A Table is giv'en, showing the electromotive force in volts pro- 

 duced 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 trans- 

 verse 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 

 isogomc 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 magnitude 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 evi- 

 dence enough to show that these currents, though certainly ex- 

 isting 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 ex- 

 aggerate the magneto-electric induction caused by their flow. 



March 15. — Dr. J. Dalton Hooker, C.B., President, in the Chair. 



The following paper was read : — 



" On the Density of Solid Mercury." By Prof. J. W. Mallet, 

 University of Virginia. 



I have lately taken advantage of a heavy fall of very cold and 

 finely pulverulent snow, well adapted to the preparation of freezing- 

 mixtures, to redetermine (with accuracy, I believe) the density of 

 mercury in the solid state and at a definite temperature. 



Fhil Mag. S. 5. Vol. 4. No. 23. Aug. 1877. L 



