for incafuring the higher Degrees of II eat, 6cc. 307 
been obliged to content myfelf with fuch meafures as my own 
kilns and the different parts of them afforded. Thus the kiln in 
which our glazed ware is fired furnifhes three meafures, the 
bottom being of one heat, the middle of a greater, and the 
top ft ill greater : the kiln jn which the bifeuit ware is fired 
furnifhes three or four others, of higher degrees of heat ; and 
by thefe I have marked my regidered experiments. But 
though thefe meafures had been fully adequate to my own 
views, which they were not, it is plain, that they could not 
be communicated to others ; that their ufe is confined to a par- 
ticular drufture of furnaces, and mode of firing ; and that, 
upon any alteration in thele, they would become ufeleis and 
unintelligible, even where now they are bed known. And, 
indeed, as this part of the operation is performed by workmen 
of the lowed: clafs, we cannot depend upon any great accuracy 
even in one and the fame furnace. It has accordingly often 
happened, that the pieces fired in the top of the kiln in one 
experiment have been made no hotter than thole fired in the 
middle in another, and vice ver/d . 
The force of fire, in its higher as well as lower dages, can 
no otherwife be judly afcertained than by its effects upon fome 
known body. Its effect in changing colours has already been 
hinted at ; and I have oblerved compofitions of calces or iron 
with clay to affume, from different degrees of fire, luch a 
number of didindt colours and fhades as promifed to afford 
ufe ful criteria of the relpective degrees. 
With this idea, I prepared a quantity of fuch a compofition, 
2nd formed it into circular pieces, about an inch in diametei, 
and a quarter of an inch thick. A numbei of thefe was placed 
in a kiln, in which the fire was gradually augmented, with as 
S f 2 m^h 
