130 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Applying to the raeau values of E, found above, certain small correc- 

 tions which take account of errors in the graduation of the therinometers, 

 we get, — 



T • E 



26°.6 1028 X 10-« volt 



ir.3 980 " " 



54°.5 936 



71°.! 870 " '^ 



These numbers plotted, with temperatures for abscissas and electro- 

 motive force per degree for ordinates, indicate a curve wliich descends with 

 very gradually increasing slope with rise of temperature. Indeed, this 

 curve is so nearly a straight line that its curvature cannot be satisfactorily 

 shown in a small figure. It would be almost perfectly straight if the 

 numbers given under E were 1028, 975, 929, and 870. 



The minute examination which I was obliged to give to the individual 

 small cylinders, during measurements of their electrical resistance, led me 

 to notice defects and possible distortions which might, I feared, have 

 affected their thermo-electric quality. Accordingly, several months after 

 tests just described were made, I undertook a similar test with ten of the 

 somewhat larger cylinders already mentioned, which had apparently suf- 

 fered much less in the process of milling. In this later test I found it 

 convenient to enclose the bars in a tube of glass instead of a tube of 

 wood. This test gave 1064 X 10"^ for E at 15°.4 C. 



The earlier tests, above described, did not run so low in temperature, 

 but by extrapolation they give, for 15°. 4 C, E = 1065 X 10~^, or some- 

 thing very close to that, a satisfactory agreement. 



Determination of the Difference op Temperature of the In- 

 going AND Outgoing Water at the Chamber above the 

 Disk. 



It has been already stated that a differential platinum thermometer 

 was used for this purpose instead of the two copper-German-silver thermo- 

 electric junctions which had been employed in the preceding investiga- 

 tions. This change was the result of a conviction, the fruit of much 

 experience and vexation, that no suitable permanent protection can be 

 found for the thermo-electric junctions against the action of hot water. 

 A thin layer of shellac well dried on appears to be the best coating ; but 

 this is liable to give way at a most inconvenient time and bring to naught 



