THERMOMETERS FOR MEASURING DEGREES OF HEAT. 343 



" Thermometers for Measuring Strong Fire, or the Degrees of Heat 

 above Ignition. 



" To those who are conversant in experimental inquiries, or in 

 the operations of manufactures and arts that are carried on by fire, 

 it is unnecessary to mention the importance of a thermometer by 

 which the force of fire in furnaces of every kind may be accurately 

 measured and appreciated, in the same denominations as the lower 

 degrees of heat are by the common thermometers. 



" Such an instrument I have now the satisfaction of offering to the 

 public. As the thermometer itself is accompanied with a pamphlet 

 explaining its construction and use, and as the results of my experi- 

 ments, both respecting its construction and the comparison of its 

 scale with that of Fahrenheit's continued, have been honoured with 

 a place in the Transactions of the Royal Society,* it will here be 

 sufficient just to mention the general principles on which it is 

 founded, viz. : that earthy bodies of the argillaceous order have 

 their bulk diminished ly fire in proportion to the degree of heat 

 they are made to undergo ; and that, consequently, the contraction 

 of this species of matter affords as true a measure for strong fire as 

 the expansion of mercury or spirit of wine does for the lower degrees 

 of heat ; but with this difference, that the contraction of the argil- 

 laceous mass is a permanent effect ; so that the degree of heat is not 

 here determined by a single transient observation made in the fire 

 itself, but its measure is preserved, and is to be examined when 

 grown cold, or at any future time. 



" The argillaceous matter is formed into equal small pieces, called 

 thermometer pieces ; and one of these, which may be conceived as 

 the detached bulb of a thermometer, is put into the fire that is to 

 be measured, either in a little case made for that purpose, or in the 

 same vessel with the subject matter of the operation. 



" A gauge, consisting of two rules fixed on a flat plate, a little 

 nearer together at one end than the other, so as to include between 

 them a long converging canal, divided on the side, serves for dis- 

 covering minute variations in the bulk of the pieces. A raw piece 

 will just enter to at the wider end of the canal : after it has been 

 in the fire, if it be gently slid along till it is stopped by the con- 

 vergency of the sides, the degree at which it stops will be the 

 measure of its diminution, and, consequently, of the heat which it 

 has undergone. 



"As the accuracy of the scale of the common thermometer 



* Phil. Transact., vols. Ixxii., Ixxiv., Ixxvii. 



