576 PHILOSOPHICAL TRANSACTIONS. [ANNO 1784. 



the extreme difficulty, not to say impracticability, of obtaining, in common 

 fires, or in common furnaces, a uniform heat through the extent even of a few 

 inches. Incredible as this may appear at first sight, whoever will follow me in 

 the operations I have gone through, placing accurate measures of the heat in 

 different parts of one and the same vessel, will soon be convinced of its truth, 

 and that he can no otherwise expect to communicate with certainty an equal heat 

 to different pieces, than by using a fire of such magnitude as to exceed perhaps 

 some hundreds of times the bulk of the matters required to be heated. To 

 such large body of fire, therefore, after many fruitless attempts in small furnaces, 

 not a little discouraging by the irregularity of their results, I at length had re- 

 course, fitting up for this purpose an iron oven, used for the burning-on of ena- 

 mel colours on earthen-ware, about 4 feet long, by 2i- wide, and 3 feet high, 

 which is heated by the flame of wood conducted all round it. An iron muffle, 4 

 inches wide, 24 inches high, and 10 inches long, containing the gage and piece, 

 was placed in the middle of this oven, and the vacancy between them filled up 

 with earthen-ware, to increase the quantity of ignited matter, and thereby com- 

 municate the heat more equably from the oven to the muffle. In such a situa- 

 tion of the muffle, in the centre of an oven more than 500 times its own capa- 

 city, it could not well fail of being heated pretty uniformly, at least through the 

 small space which these experiments required; nor have I found any reason to 

 suspect that it was not so. 



The gage being laid flat on the bottom of the muffle, with the silver piece in 

 the canal as before, some of the clay thermometer pieces were set on end on the 

 silver piece, with that end of each downwards which is marked to go foremost in 

 measuring it; that is, they were in contact with the silver in that part of their 

 surface by which their measure is afterwards ascertained. I was led to this pre- 

 caution by an experiment I had made on another occasion, in which a number 

 of thermometer pieces having been set upright on an earthen-ware plate, over a 

 small fire, till the plate became red-hot, all the pieces were found diminished, 

 some of them more than 2 degrees, at the lower ends which rested on the plate, 

 while the upper ends were as much enlarged, not having yet passed the stage of 

 extension which, as observed in the former paper, always precedes the thermo- 

 metric diminution: thus we see how punctually every part of the piece obeys the 

 heat that acts on it. 



The fire about the oven was slowly increased for some hours, and kept as even 

 and steady as possible, by an experienced fireman, under my own inspection. On 

 opening a small door, which had been made for introducing the apparatus, and 

 looking in from time to time, it was observed, that the muffle, with the adja- 

 cent parts of the oven and ware, acquired a visible redness at the same time; 

 and in the progress of the operation, the eye could not distinguish the least dis- 



