288 PROCEEDINGS OF THE AMERICAN ACADEMY. 



jacket give escape for air, and allow the use of thermometers for taking 

 the temperature of the water in the jacket. 



This water flows over as well as around the enclosed apparatus. The 

 opening in the double top of the jacket, through which extend the tubes 

 shown above Jx and J 2 in Figure 2, is about 7.5 cm. by 2.5 cm. Below 

 Ji and J.2, down to the hard rubber ring h h, the tubes and funnel were 

 thickly wrapped with cotton to lessen radiation between these parts and 

 the jacket. The space between h h and h'h', as well as that around and 

 below hHi', was carefully and fully packed with the same material. 



Figure 2 shows two fine copper wires leading out from the coating C, 

 and passing through holes in the hard rubber ring h h, where they are held 

 in place by means of hard rubber plugs, k^ and X-o, with soft rubber pack- 

 ing. There are, in fact, see Figure A, p. 290, thirteen such wires, 0.018 cm. 

 in diameter, attached to C by electrolytic deposit of copper by a process 

 sufficiently described in the article referred to in the beginning of this 

 paper. Wire no. 13 is attached at the centre of C ; nos. 3, 6, 9, and 12 

 are attached symmetrically about 2 cm. from the centre ; nos. 2, 5, 8, and 

 11, symmetrically about 3.2 cm. from the centre; nos. 1, 4, 7, 10, sym- 

 metrically about 4.4 cm. from the centre. Similar wires, nos. l'-13', are 

 similarly attached to the coating C, no. 1' being immediately beneath 

 no. 1, no. 2' immediately beneath no. 2, etc. These wires pass through the 

 ring h'h' exactly as nos. 1-13 pass through the ring /^A. To prevent 

 deposit of copper upon the free parts of the wires during the process of 

 attachment, and to prevent illegitimate metallic contacts between the 

 wires and the coatings C and C during the experiments on conduction, 

 the wires were coated with shellac between the points of attachment to 

 the coatings and the places of exit through the hard rubber rings. Out- 

 side the rings each wire was led to a point on a wooden platform, where 

 it was, by means of a screw and copper washers, held in firm copper con- 

 nection with a larger copper wire. The twenty-six larger wires thus 

 brought into connection led to a like number of small mercury wells in 

 a board j^laced at some distance from the apparatus shown in Figure 2. 



Determination of the Difference of Temperature of 

 THE Two Faces of the Disk. 



The mercury wells were so arranged that by means of copper con- 

 nectors reaching from one well to another any point of junction on the 

 upper coating of the iron disk could be thrown into circuit with the cor- 

 responding point on the under coating and with an astatic galvanometer. 



