Defcription of an AJfay-furnace. 257 



Description of the Pyrometer. 



The principle upon which this inftrumcnt is conftrufted 

 is the expanfion of metal by heat. After feveral experiments 

 with copper, iron, and fteel, I was induced to give the pre- 

 ference to the latter. Copper was found to be too eafily 

 fufed, and the fcale by that means lefs extenfive than was 

 wifhed. Malleable iron, by being heated in this manner, 

 frequently acquired permanent additional length, and pafTed 

 into the ftate of femi-fteel. This principle it derived from 

 the contact of the furrounding charcoal, introduced for the 

 purpofe of preventing oxydation from taking place upon the 

 furface of the bar. Highly cemented fteel, when expofed in 

 this manner to the greateft heat, acquired nofenfible increafe 

 of length : a bar of polidied fteel was therefore adopted. 



A correfponds with the plate of iron K in the drawing of 

 the furnace, on which the pyrometer is fupported. B an 

 upright, on which is fufpended the lever. CC the lever, 

 ten inches long from the fulcrum to the extremity, which 

 adls upon the fcale, and one inch on .the other end from the 

 fame point to a line drawn through the centre of the ex- 

 panding bar. From this it is obvious, that if the ftiort end 

 of the lever be elevated half an inch by the expanfion of the 

 bar of fteel, the other end will fufter a depreflion ten times 

 as much, or equal to five inches. Thefe I have divided into 

 50 degrees, (equal to each other on a tangent of the arch 

 that forms the fcale DD,) fo that the moft minute expanfion 

 in the bar will be vifibly indicated. Should the ratio of ten 

 to one be thought too linall, the power of the lever can eafily 

 be increafed to 13 or 20, and the fcale thus greatly enlarged. 

 When the pyrometer is at reft, the index points at 0°, and 

 its other extremity refts lightly upon the ftiarp point of the 

 fteel bar. It is therefore requifite to make the lever nearly 

 an exa6t balance, by forming its ftiort end fufficiently weighty 

 to overcome the great length of the other. The fcale being 

 carried the length of 50 degrees, allows the bar to expand 



Vol. IV. S £-ioth.. 



