US.] 
PORCELAIN AIR THERMOMETRY. 
181 
3 special requirements of the calibration problem, excepting, of course, 
3 limited scope of temperature ; but it is expedient to proceed more 
lically and introduce an entirely new and distinct furnace for the pur. 
ses in question. This I have done in a way indicated in plan in 
3 diagrammatic Fig. 30. The body of the furnace is a thick cylin- 
leal box, B B, surmounted by a hemispherical lid suitably perforated, 
this cylindrical inclosuie a spherical muffle, provided with hollow 
eral arms or axles, E and F, and placed symmetrically with respect 
the center of figure of the furnace, is free to rotate around the hori- 
atal axis of the arms. If the rate of rotation be sufficient this mech- 
ism insures constancy of temperature within the muffle around the 
rizontal E F. Two blast burners, G and E, purposely placed tangen- 
lly or diagonally, so as to be equivalent to a couple, blow a cyclone 
flame into this furnace, equalizing temperature around the central 
'tical. Virtually therefore the muffle, regarded as a geometrical 
36a. Elliptic revolving muffle; diagram. 
Fig. 36. Revolving mnfflo ; diagram. 
ere, has two rotations, one about an axis, E F, a second around the 
tical, passing through 0. To make this apparatus theoretically per- 
a third rotation around an axis passing through and perpendic- 
p to E F would have to be supplied. This third rotation is a mechan- 
impossibility, bearing always in mind that cumbersome or compli- 
ed apparatus would rather detract from the end to be attained than 
to it. The two rotations can be made to suffice. In the spherical 
ce of constant temperature thus obtained is placed the bulb of the 
thermometer, with its stem x^rojecting into and through the arm F- 
center of bulb and that of muffle as nearly as possible coincide, 
bulb is held in position and free from the muffle by a clamp attached 
tern on the outside of the furnace. The thermo-element is intro- 
ed through the opposite arm E in such a way that the junction may 
contiguous with the air thermometer. The insulating tubulure is 
supported by a clamp on the outside of the furnace. It is an essen- 
part of the construction of the present apparatus that during rota- 
the muffle touches neither the air thermometer nor the thermo- 
lent, both of which apparatus are stationary, and suspended quite 
from all parts of the furnace. 
aving thus indicated the general principles of the constant temper- 
'e apparatus, I shall next describe the practical form of this fur- 
i, which, after many trials, has been found satisfactorily serviceable. 
(835) 
