for me a faring the higher Degrees of Heat , &c. 3 1 1 
fome obfervations which occurred in the progrefs of the 
inquiry. 
I . There ought to be a certainty of the clay being eafily, 
and at all times, procurable in fufficient quantity, and on mo- 
derate terms. That this is the cale with the clay here made 
choice of, will be evident to every one acquainted with the 
natural hiflory of Cornwall, where there are beds of this clay, 
inexhauftible, and in too many hands to be monopolize:!. If 
this Ihould not prove fat is factory, the author offers to this 
illuftrious Society, and will think himfelf honoured by their 
acceptance of, a fufficient fpace in a bed of this clay to fupply 
the world with thermometer-pieces for numerous ages ; and he 
does not apprehend, that any greater inconveniences can arife 
to foreign artifts or philolophers, from their being fup- 
plied with clay for thefe thermometers from this, fpot only, 
than what we now feel from being fupplied with mercury 
for the common thermometers from the Spanifh or Hungarian 
mines. 
II. We ought to be allured alfo, that all the clay made ufe 
of for thefe thermometers is perfe&ly fimilar. For this pur- 
pofe, it will be belt to dig it out of the earth in confiderable 
quantity at once, an extent of fome fquare feet or yards in 
area, and to the depth of fix or feven yards or more from the 
furface, and to mix the whole thoroughly together, previous 
to the further preparation already mentioned. When the farfb 
quantity is exhaufted, another perpendicular column may be 
dug from the fame bed, clofe to the farff, to the fame depth, 
and prepared in the fame manner; by which means we may be 
allured of its fimilarity with the former parcel, and that it will 
diminiffi equally in the fire. 
4 
III. 
