arus.J PORCELAIN AIR THERMOMETRY. 185 
Finally, the figure shows the air thermometer// Jc i e in position, sup- 
►orted by the universal clamp m m attached to the vertical rod q q. A 
imilar universal clamp, n n, on the opposite side of the furnace supports 
he insulator of the thermo-element 1c h. The clamp n n attached to the 
od r should be a spring, so that elements may easily be either inserted 
»r withdrawn. The ends of the wires of the thermo-couple appear at 
y and fi and pass thence to a petroleum bath of known temperature, 
vmere they are suitably connected (page — ) with the terminals of the 
neasuring apparatus. The junction of the thermo element is in contact 
ither with the external surface or with the closed end of the re-entrant 
ube, according to the form of porcelain bulb selected. Of this further 
aention will be made. It is here in place to state the method of inserti- 
ng the air thermometer, a method which must be convenient and expe- 
litious. Supposing the collars N . . . , N f . . . to be removed the air ther- 
Qometer bulb is covered properly by the two halves of the muffle. The 
ollars themselves are completely cut through on one side by a slit be- 
ween two nearly contiguous axial planes, which slit passes through the 
lange P P, as shown at s n in Fig. 38, as well as through the body of 
he collar, and is quite large enough to admit the capillary platinum tube 
f the air-thermometer. This slit does not seriously weaken the collar, 
trengthened as it is by the central rib into which the screws t are sunk, 
nd by the flange P P. In this way the collar at the air-thermometer 
nd of the muffle may be slipped on quite as readily as the other. Hav- 
g therefore centered the muffle, as described above, it is then easy to 
x and center the air thermometer, so that it may be quite free from 
ontact with the muffle. In the case where a soldered air-thermometer of 
tie form Fig. 33 is used, a muffle of an axis sufficiently wide to accom- 
odate the neck must be used. A muffle of this kind is given in see- 
on in the diagram, Fig. 36. In place of adjusting the muffle it is often 
esirable to adjust the furnace. This may be done by four set screws, 
t tt, Fig. 38, which act in pairs at right angles to each other. An oil- 
ropper, by which the roller Q may be kept lubricated, is a- desirable 
ddition. Some such nonessential parts are omitted in the figure to 
revent confusion of lines. 
Remarks regarding the apparatus and manipulation. — Bearing in mind 
iat this furnace is as nearly as is practically convenient or possible 
le outcome of a theoretical principle for the construction of constant- 
iinperature apparatus, that all manipulations to be applied may be 
ade safely and with expedition, it is well to summarize the advantage 
ained, as well as to allude to such others as are easily within reach. The 
laximuin temperature obtainable is indefinitely high, much higher than 
in be defined by porcelain air-thermometer measurements. Any other 
imperatives below this extreme value can be obtained with requisite 
Dnstancy. If these intermediate temperatures be in the region of red 
eat it is simply necessary to diminish the supply of gas by partially 
losing the gauge stop-cock. If the temperatures be below red heat 
(839) 
