Philosophical Transactions. 339 



universal application both to scientific researches, and to various 

 purposes of art. He introduces the subject by an account of the 

 attempt of M. Guyton de Morveau, to employ the expansions 

 of platiria for the admeasurement of high temperatures, and for 

 connecting the indications of Wedgewood's pyrometer with the 

 mercurial scale, and verifying its regularity. The experiments of 

 that philosopher, on the contraction of porcelain in actual compa- 

 rison with the platina pyrometer, were extended to no higher tem- 

 perature than the melting point of antimony ; but they are sufficient 

 to establish the existence of a great error in Wedgewood's original 

 estimation of his degrees up to that point. This he carries on by 

 calculation, on the hypothesis of uniform progression of expansion, 

 up to the melting point of iron ; the construction of his instrument 

 not admitting of its application to higher temperatures than a red 

 heat, in which platina becomes soft and ductile. 



Mr. Daniell shews, by an examination of M. Guyton's results, 

 that he has failed in establishing the point he laboured to prove, 

 namely, the regularity of the contraction of the clay pieces* 



The pyrometer of the author consists of two distinct parts, the 

 one designated the Register; the other the Scale. 



The first is a square tube of black lead, eight inches long, cut out 

 of a common crucible of the material, closed at one end, and having 

 at the other a portion of about six tenths of an jnch in length, cut 

 away to the depth of half the diameter of the bore, so as to leave a 

 shoulder near the end. A bar of any metal, six inches and a half 

 long, is introduced into the cavity, resting against its solid end, and 

 a cylindrical piece of porcelain, about one and a half inch long, 

 which he calls the index, is placed upon the top of the bar, and 

 projects beyond the open part of the tube, being confined in its 

 place by a ring or strap of platina passing round it, and also round 

 the end of the black lead bar, and made sufficiently tight by a small 

 porcelain wedge inserted between them. When the instrument 

 thus prepared is subjected to heat, the porcelain index will be 

 forced up by the expansion of the metallic bar, to a certain dis- 

 tance, where it will remain when the bar retires from it, on cooling. 

 The distance it has been moved from its original position, will be 

 the measure of the difference of expansion of the metallic bar, and 

 of an equal length of the black lead, in which it is contained. This 

 cannot be influenced by any permanent contraction which the black 

 lead may undergo by intense heat ; because any such contraction 

 will occur at the moment of the greatest expansion of the metal ; 

 and the index will still mark its point of furthest extension upon 

 this contracted basis. It remains then to measure accurately the 

 distance to which the index has been moved, by the application of 

 the scale, which is a detached instrument constructed of two rules 

 of brass, joined together at a right angle, the one fitting square 

 upon two sides of the black lead bar, the other resting on its 

 shoulder; with these are connected two arms, which, acting on 

 the principle of proportional compasses, measure the distance of the 



