3 ° 9 
for meafuring the higher Degrees of Heat , &c. 
of experiments lately made with this view, that it is a more 
accurate and extenlive mealure of heat than the different fhades 
of colour. 
I have found, that this diminution begins to take place in a 
low red-heat ; and that it proceeds regularly, as the heat in- 
creafes, till the clay becomes vitrified, and confequently to the 
utmoff degree that crucibles, or other veffels made of this ma- 
terial, can lupport. The total contraction of lome good clav r s 
which I have examined, in the ftrongefi: of my own fires, is 
confiderably more than one-fourth part in every dimenfion. 
If, therefore, iwe can procure at all times a clay fufficiently 
apyrous or unvitrefcible, and always of the fame quality in 
regard to contraction by heat; and if we can find means of 
meafuring this contraction with eale and minute accuracy, I 
flatter myfelf, that we lhall be furnilhed with a meafure of 
fire fufficient for every purpofe of experiment or bufinefs. 
We have, in different parts of England, immenfe beds of 
clay ; each of which, at equal depths, is pretty uniform in 
quality throughout its whole extent. Of all the forts I have 
hitherto tried, fome of the pureff Cornifh porcelain clays feem 
the bell: adapted, both for fupporting the intenfity, and mea- 
furing the degrees, of fire. 
For preparing and applying this material to thermometric 
purpofes the following method is propol'ed. 
The clay is firft to be walked over, and, whilft in a dilute 
ffate, palled through a fine lawn. Let it then be made dry, 
and put up in boxes *, 
* While the clay is thus kept dry in boxes, as well as while it continues in its 
natural bed, it is fecure from alterations in quality, which clays in general are 
fubjeft to undergo, when expoled, for a long courfe of years, to the joint actions 
of air and moilture.— In the lawns I made ufe of, the interfaces were each lefs 
than the ioc,coo part of an inch. 
The 
