BY COMPRESSION". 



401 



firmnefs, and, lufienng no chanee in very low heats, excluded Continuation of 



irrii X ■ , ^ n n r , • t the method of 



the Tiirible metal m the firft uages or the experiment. experiment. 



The large lube, thas filled, was placed in the cradle, fome- 

 times with the muzzle upwards, and foraetimes the reverfe. I 

 have frequently altered my views as to that part of the ar- 

 rangement, each mode pofleffing peculiar advantages and dii- 

 ad vantage?. With the muzzle upwards, (as (hewn in Fig. 24 and 

 25), the beft fecurity is afforded againft the intrufion of the fufi- 

 ble metal ; becaufe the air, cjuitting the air-tube in the working 

 pofition, occupies the upper part of the barrel; and thefufible 

 metal ftands as a liquid (at ^, Fig. 25.) below the muzzle of the 

 lube, fo that all communication is cut off, between the liquid 

 metal and the inlide of the tube. On the other hand, by this 

 arrangement, the fmaJI tube, which is the fiducial part of the 

 apparatus, is placed at a confiderable diftance from the breech 

 of the barrel, fo as either to undergo lefs heat than the upper 

 part, or to render if nece(rary that the barrel be thrufi high into 

 the muffle. 



With the muzzle of the large tube downwards, the inner 

 lube is placed (as fliewn in Fig. 22), fo as ftill to have its 

 muzzle upwards, and in contact with the breech of the large 

 tube. This has the advantage of placing the fmall lube neair 

 to the breech of the barrel : and though there i;; here lefs fe- 

 GUfity againft the intrufion of liquid metal, I have found that 

 a point of little confequence ; fince, when the experiment is 

 a good one, and that tiie carbonic acid has been well con- 

 fined, the intrufion feldom lakes place in any pofition. In 

 whichever of the two oppofite pofitions the large tube was 

 placed, a pyrometer was always introduced, fo as to lie as neat 

 as poflible to the fmall tube. Thus, in the firfl-raentioned 

 pofition, the pyrometer was placed immediately below the 

 large tube, and, in the other pofition above it ; fo that, in both 

 cafes, it was feparated from the carbonate by the thicknefs 

 only of the two tubes. 



Much room was unavoidably occupied by this method, 

 which necefTarily obliged me to ufe fmall quantities of car- 

 bonate, the fubje6t of experi;nent feldom weighing more than 

 10 or 12grains^ and in others far lefs*. 



On 



* I me-ifured the capacity of the air-tubes by means of granu- 

 lated tin, afting as a line and equal fand. By comparing the weight 



Of 



