. The Philosophical magazine; a journal of theoretical, experimental and applied physics . ched to a prismatic stand, 2 metres high, consisting offour uprights rigidly joined at tin; top and the l) are made for (icpial heights of the nuncury inboth branches, by the cathetometer. A barometer is attachedto the stand. Sensitive thermometers at the lower and theupi)er end of the glass measuring-tube show its temperature. <y. § 18. * First u^^ed by Rcgnavilt {Rcldtion c/es J^xp/-r/^nrcs, Piiris. 1847, p. 204). 6 Fusion Constants of Igneous RocJc. 8. Revolving Muffle.—In order t
. The Philosophical magazine; a journal of theoretical, experimental and applied physics . ched to a prismatic stand, 2 metres high, consisting offour uprights rigidly joined at tin; top and the l) are made for (icpial heights of the nuncury inboth branches, by the cathetometer. A barometer is attachedto the stand. Sensitive thermometers at the lower and theupi)er end of the glass measuring-tube show its temperature. <y. § 18. * First u^^ed by Rcgnavilt {Rcldtion c/es J^xp/-r/^nrcs, Piiris. 1847, p. 204). 6 Fusion Constants of Igneous RocJc. 8. Revolving Muffle.—In order that corresponding indica-tions of thermoconple and air-thermometer may he compared,both must be placed in a space at practically imiform tem-perature, which must be variable at ])leasure from ordinarytemperatures to extreme white heat. I constructed a specialfurnace for this purpose, in the way indicated in plan in thediagram (fig. 4). The body of the furnace is a thick cylin-drical box, B B, surmounted by a hemispherical lid, suitablyFig. 4.—Plan of the revolving muffle. perforated. In this cylindrical enclosure a spherical muffle,C D, provided with hollow lateral arms or axles, E F, andplaced symmetrically with respect to the centre of figure, isfree to rotate around the horizontal axis of the arms. If therate of rotation be sufficient, this insures constancy of tempera-ture within the muffle around the horizontal E F. Two blast-burners, G and H, purposely placed tangentially so as to beequivalent to a force-couple, blow a vortex of flame into thisfurnace, equalizing temperature around the vertical. Vir-tually, therefore, the muffle, regarded as a geometrical sphere,has two rotations, one round E F, and the other round thevertical through 0. To make this apparatus theoreticallyperfect a third rotation, round a horizontal axis through 0,and perpendicular to E F, would have to be supplied. Thisthird rotation is a mechanical impossibility, bearing alwaysin mind th
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