3 9 ^ Mr. W e d g wood’s additional Qbferhiatims on 
By due attention to the circumftances above ftated, any fingle 
quantity of clay may be made up into thermometer-pieces, that- 
ihall differ very little, if any thing at all, from one another. 
But a new difficulty now arofe, more embarraffing than any 
of the former ; that of procuring frefh fupplies of clay, of 
the fame thermometric'quality with the firft. The quantity 
of the clay which, after trial of many others, 1 had made choice 
of, was fmall ; but the particular fpot it was taken from being 
known, and having purchased the little eftate in which it was 
raifed, I had not a doubt of obtaining more of the fame when 
it fhould be w r anted : for clays in general, when raifed from an 
equal depth, in the fame ftratum, and near the fame place, 
are found to poflefs the fame properties, with refpeft to ducti- 
lity in the hands of the workman, a difpofition to affume by 
fire a porcelain or vitreous texture, fingly or in compofition, 
and all other qualities relative to their ufe in pottery. In this, 
however, I was deceived ; for when a frefh fupply was wanted, 
to complete my experiments, though I had fome taken from a 
pit joining to the firft, and at the fame depth, it was found to 
diminifh differently from the former parcel. 1 then had fome 
raifed from different parts of the fame field and bed, and at 
different depths ; and in various other places in Cornwall* from 
the fpot where this fpecies of clayis firft met with to the Land’s^ 
End; but all th.efe clays differed fo much from the firft in the 
quantity of their diminution by fire, and moft of them like- 
wife from each other, that I, defpaired of being ever able to 
find one that wmuld correfpond with it, or any natural clays 
that could be obtained twice of exadlly the fame thermometric 
properties, how fimilar foever in other refpeCts. 
Upon a review of the numerous comparifons which I have 
made of thefe new clays, indifferent degrees of heat, from the 
com- 
