404 Mr . Wedgwood’s additional Obfervations oft 
venience originally imagined to attend it), but may be pro- 
cured and prepared in all parts of the world where good 
common clay, and alum, are to be found; and correfponding 
thermometers may, confequently, be conftrufted, without 
any ftandard to copy from. For, if a converging canal be 
formed, of any convenient length, with the widths, at the 
two ends in the proportion of 5 to 3, with the (kies perfedlly 
ftraight, and divided into 240 equal parts, numbering the 
divifions from the wider end^ and if a clay be obtained of 
fuch quality, that when formed, in the manner already men- 
tioned, into pieces of fuch fize as to enter to o in the gage or 
canal, thefe pieces fhall juft begin to diminiih,. or. go a little 
further in the canal, by a heat vifibly red ; — -go to 27, by the 
heat in which copper melts ; — about 90 by the welding heat 
of iron; about 160, by the greateft heat that can be produced 
with coaked pit* coal in a well conftrudled common air-furnace, 
about eight inches fquare, ftill continuing bibulous, fo as to 
flick to the tongue : fuch gages, and pieces of fuch clay, fo 
«fcljuftsd, will always compofe correfpondent thermometers. 
Having mentioned occafionally feveral alternate periods of 
dilatation and cootradlion in clay, it may be proper to ftate,. 
and bring into one view* the. whole fucceflion of changes 
which. I have obferved in this curious material ; as otherwife 
they might create fome confufion in the minds of thofe who 
have not had occafion- to think attentively on this fubjedl, and 
lead them to afk how a body fo variable, and liable to fuch 
oppofite changes from different degrees, of heat, can yet be a 
juft m.eafure of thofe degrees. 
* Or the divifions on the fide may be continued to 30O'; and in that cafe, 
inflead of the widths of the two ends being in proportion of the odd numbers. 
$ and g, the one will be juft double to the other. 
The 
