^^'i On Mr, Wedgwood's Pj'romeief\ 



into inches and tenths, 'j'he pyrometer pieces are made of 

 a particular, kind of Cornifli clay by means of moulds, and 

 are pared afterwards, when dry, by a paring-gauge, to infure 

 their being all of one fize as to length: a variation in the 

 other dimenfions is of no eonfeqaence, as it is by the length 

 their Ihrinkuge is to be afterwards incafured. 



By means of a linjilar gauge, but by employing the expan- 

 fion of a piece of fdver by heat, Mr. Wedgwood fucceede^ 

 in obtaining a knowledge of the intermediate degrees of heat 

 between that of boiling mercury and the zerq of his own. 

 fcale; from which he afcertained, that one degree of his was 

 ecjual to 130'^ of Fahrenheit's fcale, and that the zero of his 

 correfpondcd to 1077I F. Confequently, to accommodate 

 the refults obtained by the pyrometer to Fahrenheit's fcale, 

 ail that is neceflary is to multiply the pyrometric degrees 

 by 130, and to the produ£l to add 1077 ~. 



It is pretty generally believed that the pyrometer pieces 

 which have of late been prepared, do not give the fame re- 

 lults with thofe which were firil made by Mr. Wedgwood; 

 a circumftance which makes it extremely delirable that the 

 quantities and kinds of earth, neceflary to form fuch rolls aS 

 will always give correal refults, fliould be accurately afcer«^ 

 tained. 



From Mr. Wedgwood's own experiments, it appears that 

 the clay he made ufe of in the conftrudlion of his firft pieces 

 coniifted of two parts of pure liliccous earth to three of 

 argil *. 



.C Vauquclin> who has analyfed Mr. Wedgwood's pyro- 

 meter pieces t, ftates, that they contain 64*3 parts of pure 

 filcx, 25 parts of argil, 6 of lime, o'2 oxyd of iron, 6'Z of 

 water.. 'J'hefe quantities, confequently, with an addition of 

 water to convert the whole into a pafle, fliould ferve for 

 making pyn^mtter pieces ec^ual in powers to thofe of Mr. 

 Wedgwood. 



; Whether C. Vauquelin analyfed new or old pieces, that 

 he- obtained refults fo ditlerent from thofe of Mr. Wedgwood ; 

 whether he be aware that there is any difference between 



'■* Phihfophical TranfaSlioKS I'lZi. 



.f Sec Po'ilofo^hlcal Mogawie-y \^\. V. p*. 49(5. 



them •, 



