282 Of the Propagation of Heat 



thick ; and on the flat and even surface of the ice I placed 

 a circular plate of the thinnest tin I could procure, near 

 (y\ inches in diameter, or sufficiently large just to cover 

 the ice. This plate of tin (which, to preserve its form, 

 or keep it quite flat, was strengthened by a strong wire 

 which went round it at its circumference) had a circular 

 hole in its center, just two inches in diameter, and it was 

 firmly fixed down on the- upper surface of the cake of 

 ice by means of several thin wooden wedges which passed 

 between its circumference and the sides of the jar. 



A second circular plate of tin, with a circular hole in its 

 center two inches in diameter, and in all other respects 

 exactly like that already described, was now placed over 

 the first, and parallel to it, at the distance of just one 

 inch, and like the first was firmly fixed in its place by 

 wooden wedges. 



These perforated circular plates being fixed in their 

 places, the jar was placed in a room where Fahrenheit's 

 thermometer stood at 34°, and ice-cold water was poured 

 into it till the water just covered the upper plate, and 

 then the jar was filled to within half an inch of its brim 

 with boiling water, and, being covered over with a board, 

 was suffered to remain quiet two hours. 



At the end of this time the water, which was still 

 warm, was poured off, and, the circular plate being re- 

 moved, the ice was examined. 



A circular excavation just as large as the hole in the 

 tin plate which covered the ice (namely, two inches in 

 diameter), arid corresponding with it, perfectly well de- 

 fined, and about -j^ of an inch deep in the center, had 

 been made in the ice. 



This was what I expected to find ; but there was some- 

 thing more which I did not expect, and which, for some 



