128 PROCEEDINGS OP THE AMERICAN ACADEMY. 



such cells, which differed from each other iu electromotive force about 

 1 part in 500. 



The mean difference of temperature of the two streams of water on 

 entering the apparatus was about 8° at low temperatures and about 7°. 6 

 at high temperatures. The mean difference of temperature between the 

 two surfaces of contact of the iron and copper, as indicated thermo- 

 electrically, was about 1°.42 at low temperatures and about ]°.58 at high 

 temperatures, which shows that heat is communicated more readily from 

 the water to the copper coatings, and vice versa, at high temperatures 

 than at low temperatures, other things being equal. 



In order to determine the difference of temperature, just mentioned, 

 between the surfaces of contact of the copper with the iron disk, it was 

 of course necessary to find the thermo-electromotive force corresponding 

 to a known difference of temperature of two similar junctions. This 

 datum was obtained by the method described in Appendix I of the paper 

 on the " Conductivity of Cast Iron," to which paper a number of refer- 

 ences have already been made in this writing. Some jDarticulars of the 

 present case follow. 



A second disk about 2.5 cm. thick was cut from the same end of the 

 same cylinder of Yorkshire iron that had furnished the disk already de- 

 scribed. This second disk was then cut into two half disks, and from one 

 of them was taken a slice, thickness-wise, about 11 cm. long, 2.5 cm. 

 wide, and 0.3 cm. thick. This slice was then sawed up, crosswise, into 

 thirty-three bars. All but ten of these bars were then reduced by filing 

 and milling to a diameter of about 0.16 cm., and the length of each was 

 reduced to 2.0 cm. The remaining ten were reduced in the same way 

 to a thickness of 0.23 cm. and a length of 2 cm. The bars were then 

 numbered from 1 to 33 in the order of their original position in the slice 

 from which they had been cut. Numbers 1, 4, 7, 10, 13, 20, 23, 26, 29, 

 and 32 were placed end to end, in the order just given, in the bore of the 

 wooden cylinder of the thermo-electric test apparatus shown in Figure 5 

 of the article on " Conductivity of Cast Iron." No essential change has 

 been made in this apparatus or in the manner of using it since the de- 

 scription given in the article just mentioned was written, except in these 

 particulars, that the electrical resistance * of the end-to-end row of bars 



* This resistance was likely to be as much as 2.5 or 3 ohms when the pres- 

 sure, about 3 kgms., was newly applied to the row, but a gentle rocking, from side 

 to side, of the copper blocks, kept well seated, would in tlie course of a few min- 

 utes reduce this resistance to less than 1 ohm, the end pressure remaining un- 



