136 fHILOSOPHICAL TRANSACTIONS. [aNNO 1786. 



Mercury's limb in contact with the sun's limb 20^ 45"* 37* 



Mercury bisected by the sun's limb 20 47 17 



Mercury quite out, or last contact 20 49 22 



XXII. Additional Observations on making a Thermometer for Measuring the 



Higher Degrees of Heat. By Mr. Josiah Wedgwood, F. R. S., and Potter 



to her Majesty, p. SQO. 



Inmy first paper I communicated every thing thatexperience had then taught me> 

 respecting both the construction and use of this thermometer ; but more exten- 

 sive practice has since convinced me, that other managements and precautions 

 are necessary, in order to bring it to the perfection it is capable of receiving : 

 for pieces made of the same clay, and exactly of the same dimensions, have been 

 found to differ in the degree of their diminution by fire, in consequence of 

 some circumstances in the mode of their formation, at that time unheeded, and 

 very difficult to be developed. 



Of the 2 ways proposed for forming them, the mould and the press, the 

 former was made choice of, as being, for general use, the most commodious. 

 The soft clay was pressed into a square mould with the fingers ; and the pieces, 

 when dry, were pared down on two opposite sides, by means of a paring gage 

 made for that purpose, so as to pass exactly to 0° at the entrance of the con- 

 verging canal of the measuring gage. But the pieces thus formed have been 

 found liable, in passing through strong fire, to receive a little alteration in their 

 figure, which produces an uncertainty with respect to their subsequent measure- 

 ment : the two sides, instead of continuing flat, become concave ; the edges, 

 both at top and bottom, projecting beyond the middle part, sometimes very 

 " considerably, as at a and b, fig. 7, pi. 1, where ab represents a perpendicular 

 section of an unburnt piece, and ab a like section of the same piece after it has 

 undergone a heat of 1 60 degrees. This irregularity in the form, which is sen- 

 sible only after passing through the high degrees of fire, was observed in some 

 of the early experiments, but was not then considered as productive of any 

 error. 



On more attentively examining this matter, it appeared, that when the clay 

 is pressed into a mould, the surface in contact with the mould acquires a more 

 compact texture than the inner part of the mass ; that this compactness re- 

 strains, in some degree, its diminution in the fire ; and therefore, that when 

 this surface, or less diminishable crust, is pared off from the two sides only, the 

 piece may be considered as having its upper and lower strata, aa and bb, com- 

 posed of a less diminishable matter than the intermediate part, the necessary 

 consequence of which structure will be such a figure as we find the pieces to 

 assume ; for if any stratum in the mass shrinks less than the rest, the extremi- 

 ties of that stratum must be left proportionably prominent. That this was the 



