Mr. Sabine on the Conducting Power of Mercury, 457 



12*5 1" 

 successive blows = -^- = — ; and therefore a second blow 



will occur before the girder has begun to recover itself, and will 



therefore still further increase the deflection ; a third blow and 



others also will follow before the girder has begun to recover its 



former position, and these effects being accumulated may become 



dangerous. The blows will grow feebler and feebler after the 



first, because each is produced by the impact of only that portion 



of the train which has not at the moment passed the point of 



interruption. 



If the velocity of the train be 20 miles (the sudden change in 



the gradient still being 1°), the intervals of the blows will be 



2" 1" 



about — instead of -=-. The result will be that the second blow 

 5 7 



will occur just as the girder has sprung back to its original 



position with a momentum equal to that which the first blow 



gave to it, but in the upward direction ; the second blow will 



therefore be counteracted by this. The third blow will then 



have its full effect; but being feebler than the first, will not 



produce so great a depression as before; and the successive 



results will be smaller and smaller. 



The calculation in this paragraph is sufficient to show the 

 great importance of the rail over the bridge being entirely free 

 from every impediment or check of every kind, and the advan- 

 tage of having the girder unattached to the roadway. 



J. H. Pratt, 



Calcutta, March 12, 1862. 



LXIL Some remarks on a Paper by Dr. A. Matthiessen, F.R.S., 

 and C. Vogt, Ph.D., " On the Influence of Traces of Foreign 

 Metals on the Electric Conducting Power of Mercury." By 

 Kobert Sabine, Esq.* 



IN the above paper, published in the March Number of the 

 Philosophical Magazine, Drs. Matthiessen and Vogt give 

 six Tables, with data and calculations of their tests, of the con- 

 ducting powers of amalgams of bismuth, lead, tin, zinc, gold, and 

 silver, in different proportions. 



Accompanying these Tables, however, no formula is given to 

 indicate how the numbers in the seventh columns, headed 

 " conducting powers calculated," are obtained. 



Having recently had occasion, in the laboratory of Dr. 

 "Werner Siemens, at Berlin, to be occupied with inquiries on the 

 electrical resistances of amalgams, I was interested in going 



* Communicated by the Author. 



