14 EFFECTS of HEAT 
have frequently altered my views as to that part of the ar-~- 
rangement, each mode poffefling peculiar advantages and dif- 
advantages. 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, quitting the air-tube in the working 
pofition, occupies the upper part of the barrel; and the fufible 
metal ftands as a liquid (at g, 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 {mall 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 neceflary that the barrel be thruft high into the 
muffle. 
Wirn the muzzle of the large tube downwards, the inner 
tube is placed (as fhewn in fig. 22.), fo as ftill to have its 
muzzle upwards, and in conta¢t with the breech of the large 
tube. This. has the advantage of placing the {mall 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 jis 
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 feparated from the carbonate by the thicknefs 
only of the two tubes. 
Mucn room was unavoidably occupied by this method, 
which neceflarily obliged me to ufe fmall quantities of car- 
bonate, 
