. The London, Edinburgh and Dublin philosophical magazine and journal of science. of the gauge and other parts of the apparatusperfectly dry. P is a very elaborate cathetometer for observingthe mercury columns in B and C. This beautiful instrumenthas a revolving column with vertical scale, and vernier withmicroscope, reading to hundredths of a millimetre. The eye-piece micrometer reads directly to hundredths of a millimetre,and the divisions on the revolving head of the screw are soopen, that tenths of divisions are easily and certainly estimatedby an experienced eye ; thus permitting the micr
. The London, Edinburgh and Dublin philosophical magazine and journal of science. of the gauge and other parts of the apparatusperfectly dry. P is a very elaborate cathetometer for observingthe mercury columns in B and C. This beautiful instrumenthas a revolving column with vertical scale, and vernier withmicroscope, reading to hundredths of a millimetre. The eye-piece micrometer reads directly to hundredths of a millimetre,and the divisions on the revolving head of the screw are soopen, that tenths of divisions are easily and certainly estimatedby an experienced eye ; thus permitting the micrometer to beread directly to thousandths of a millimetre. Of course thecathetometer is perfectly located, not as shown, but with theobjective of its telescope equally distant from the axes of thetubes B and C when it is alternately directed to them; andat such a distance that its micrometer readings correspond toa millimetre scale. The whole apparatus is located in a base-ment room, on a stone floor, whereby vibrations are reducedto a minimum. of Small Gaseous Pressures. 417i. cr. Phil. Mag. S. 5. Vol. 44. No. 270. Nov. 1897. 2 H 418 Mr. C. F. Brush on the Measurement The most important part of the gauge is the head B. Thepurpose of its great diameter is the reduction of capillarydepression in its mercury column. But its size necessitates avery close approach of the mercury to its upper end, in orderto sufficiently reduce its capacity. Yet the remaining spacemust be measurable by the cathetometer with the utmost pre-cision. Hence the glass must not be distorted by heating,and the closed end just over the mercury must be sharplydefined. In constructing this part of the apparatus, I selecteda piece of heavy tubing which would just slip inside of B,with the least possible clearance. One end of this tube wasclosed as squarely as possible by fusion, and then ground withfine emery and a suitable tool to a convex spherical surface oflong radius. Care was taken to make the centre of cu
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