*bus.J PORCELAIN AIR THERMOMETRY. 183 
s intermittent, and in this respect objectionable. It will be well to 
state that the flame issuing at the mouth of the burner when the blast 
s cut off (a great torch fully two feet in length) is reduced to a blue 
one scarcely eight inches in length for the maximum supply of gas. 
Placed in the • position given the burners during the heating do not 
nelt, but merely grow red hot at the mouth, and the oxidation is 
minimum, because the general tendency is to reduce. It is well, 
lowever, to attach a reservoir of water 'behind the furnace and to tap 
t through lead piping and small faucets, so as to fall drop by drop 
ipon the burners and thus prevent all possibility of superheating. In 
ione of my experiments did ferruginous fluxing of the furnace body 
)ccur. I found in the experiments that to secure the maximum tem- 
>erature desirable (say 1,400°), it was not necessary to open the half- 
uch clear-way cocks more than one-third. From this downward the 
ntensity may be diminished to the merest ribbon of flame, sending in 
eality only a vortex of hot air through the furnace. Indeed this ad- 
ust ment is very satisfactory, so that with an accurately graduated 
ire attached to the cocks it is possible, after the necessary preliminary 
neasurements have been made, to open it in such a way as to strike 
^ny given temperature with some nicety. The cupola A A, which can 
>e lifted off from the body B B, along the plane A A, emphasized in Fig. 
18, has a suitable handle arrangement attached to it, which is omitted 
n the figure. 
Having thus given a furnace which can be heated to any reasonable 
legree of temperature with extreme ease and convenience, I proceed 
ext with the description of the revolving muffle. The muffle proper 
s shown at E G D F, in Fig. 37, and consists of two identical halves 
f refractory fire-clay, each of which is a hemisphere with two dia- 
aetrically opposite guttered arms. The two halves are placed together, 
rith their plane faces contiguous, but without cement. They are held 
ogether by surrounding their ends with appropriate collars of iron, 
VN N N and N' N' N' N', the outer edges of which are widely flanged. 
?hese flanges, P P and P' P', are turned circularly, with their circum- 
ferences carefully beveled, so as to fit nicely into the grooves of two 
iairs of friction-rollers, Q Q and Q' Q f , of which E E and E' E' are 
he respective axes. Here I may well say that it is difficult to bake 
he muffle in such a way that the plane faces are not warped. More- 
ver, the two tubes, when placed together, show rather an elliptic ring- 
haped section than a circular one, as is represented, with a little ex- 
aggeration perhaps, at //, Fig. 38. But this irregularity furnishes an 
xceedingly satisfactory way of fastening the muffle into the collar. 
<"or if the width of the bore be so chosen as to fit snugly on the major 
iiameter of the axle of the muffle, a flat i/-\)-shaped spring of 'steel may 
^e inserted in each narrow space between axle and collar, against the 
»ulge of which (spring) set-screws 1 1 t t, sunk into an equatorial rib 
f the collar, press as firmly as is permissible; or this space may be 
(837) 
