132 PROCEEDINGS OF THE AMERICAN ACADEMY 



ductor is the almost pure silicious earth, dug in Keene, N. H., and 

 known as " infusorial earth." With it, we were abundantly supplied 

 by the kindness of Messrs. J. A. Wriglit and James H. Wilson of 

 Keene. It is in every way suited to work of this kind, being clean, 

 free from any appreciable amount of moisture, and an almost perfect 

 non-conductor of heat. 



Upon the ends of the copper rod, covered above and below to with- 

 in 2 or 3 cm. of the edges, rests a sheet of No. 11 boiler-plate iron, 

 1.5 meters long and .9 meters wide. 



The first thing necessary to the success of our experiments with the 

 plate was that the head of gas should be constant. We found tlie 

 variation to be insensible, although a change in pressure of a fraction 

 of one mm. of water could have been detected. 



The temperature of the air must also be constant. To secure this, 

 we wei-e obliged to use the precautions mentioned above. The air for 

 the lamps entered at the bottom of the cases, and passed out from both 

 by an iron pipe placed within a wooden one. The tin cylinders were 

 filled with "infusorial." The variation in the temperature of the air 

 was less than one degree during the day. It would have been impos- 

 sible to keep the temperature exact to the x^s'^ir ^^ * degree, as is 

 reported of Biot. 



No work could be done until the plate reached a final state. For 

 this an average of five hours was required. After this time the tem- 

 peratures were sensibly constant. 



The large size of the plate was necessary, because observations 

 taken near the edge ought not to be relied on. and because the varia- 

 tion in temperature was quite rapid, whilst even a slight error in deter- 

 mining a point on the curve produced a great change in its equation. 

 For example, in one case, in a curve whose loop was about '210 mm. 

 in diameter, a change of ^oi<r millimeters in the position of a given 

 point changed 7ii in the equation r^r.^"" = c from — .5 to -]- .8, or the 

 equation from 



— r = c to r,r.,5 = c. 



In another case, a change of three millimeters altered the equation from 



r-^r^ = c to rj7^i = c 



The measurements were made with the galvanometer and thermo- 

 pile. Our galvanometer has four coils, whose resistance is 11. ohms, 

 and a mirror of 1 meter focal length. Each scale-division is f mm. 



