396 Mr\ W e dg wood ’s additional Obfervatwns on 
made. Iu one period of tlue courfe of my experiments, X 
employed, for firing them, a final!, fhallow, cylindrical veffel 
(fig. 3,) fetting the pieces on end, clofe together, on its bot- 
tom, and placing it in the middle of the fuel, in a common 
air-furnace, with care to keep the fire as equal all round it as 
poffible. It was expe£ted, that all the pieces would receive an. 
equal heat; and as they were found, after the operation, to* 
differ in their dimenfions, fometimes confiderably, from one 
another, thefe differences proved a fource of much perplexity^ 
till it was difcovered that the pieces had really undergone 
unequal degrees of heat. 
X11 the paper on the comparifon of this thermometer with 
Fahrenheit’s*, I have taken notice of the great difficulty of 
obtaining, in fmall furnaces, a perfectly equal heat, even 
through the extent occupied by. a few of thefe little pieces : 
and how different the heat may be in different parts of one 
veflei, we may be fatisfied by an eafy experiment, viz. fetting 
a cylindrical rod of clay, of the length of eight or ten inches, 
upright in the middle of a crucible, and urging it with ftrong 
fire in a common final T furnace ; the rod will be found very 
differently diminished at different parts of its height; and if 
its height he fuffieient to reach fome way above the fuel, 
nearly the whole range of the thermometric fcale may be pro- 
duced in one rod ; an ocular proof, not only of the diverfity 
of heat within a fmall compafs, but likewife, of the peculiar 
fenfibility of this thermometer, every part of the mafs ex~ 
preffing diftinftly the degree of heat which it has itfelf felt. 
It will be proper, in this experiment, to have a tube fixed in 
the bottom of the crucible, for keeping the rod fteady, as at 
fig. 4. By this means the heat of my air-furnace renders a 
* Philofophical Tranfa&ions, v©l» LXXIV* 
rod 
1 
