ii4 EFFECTS of HEAT 



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

 rangement, each mode pofieilmg peculiar advantages and dis- 

 advantages. With the muzzle upwards, (as fhewn in fig. 24. and 

 25.) the beft fecurity is afforded again!! the intrufion of the fufi- 

 ble metal j becaufe the air, quitting the air-tube in the working 

 polition, occupies the upper part of the barrel ; and the fufible 

 metal Hands as a liquid (at q, fig. 25.) below the muzzle of the 

 tube, fo that all communication is cut off, between the liquid me- 

 tal and the infide of the tube. On the other hand, by this arrange- 

 ment, the fmall tube, which is the fiducial part of the appara- 

 tus, 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 it neceffary that the barrel be thruft high into the 

 muffle. 



With the muzzle of the large tube downwards, the inner 

 tube is placed (as fhewn in fig. 22.), fo as Hill to have its 

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

 tube. This has the advantage of placing the fmall tube near 

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

 curity 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 the carbonic acid has been well con- 

 fined, the intrufion feldom takes 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 near 

 as poilible to the fmall tube. Thus, in the firft-mentioned 

 pofition, the pyrometer was placed immediately below the 

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

 cafes, it was Separated from the carbonate by the thicknefs 

 only of the two tubes. 



Much room was unavoidably occupied by this method, 

 which neceffarily obliged me to ufe fmall quantities of car- 

 bonate, 



