732 



SCIENCE. 



[N. S. Vol. VII. No. 178. 



carriage, so that it may swing out of the 

 fork when the carriage is lowered. L is a 

 pinch-cock with screw for regulating the 

 flow of mercurj', or stopping it altogether, 

 while pumping out the trap D. N is a 

 hulb containing phosphorus pentoxide, to 

 keep the interior of the gauge and other 

 parts of the apparatus perfectly dry. P is 

 a very elaborate cathetometer for observing 

 the mercury columns in B and C. This 

 beautiful instrument has a revolving column 

 with vertical scale, and vernier with mi- 

 scrope, reading to hundredths of a milli- 

 meter. The eye-piece micrometer reads 

 directly to hundredths of a millimeter, and 

 the divisions on the revolving head of the 

 screw are so open, that tenths of divisions 

 are easy and certainly estimated by an ex- 

 perienced eye, thus permitting the microm- 

 eter to be read directly to thousandths of a 

 millimeter. Of course the cathetometer is 

 permanently located not as shown, but with 

 the objective of its telescope equally distant 

 from the axes of the tubes B and C, when 

 it is alternately directed to them, and at 

 such a distance that its micrometer read- 

 ings correspond to a millimeter scale. The 

 whole apparatus is located in a basement 

 room, on a stone floor, whereby vibrations 

 are reduced to a minimum. 



The most important part of the gauge is 

 the head B. The purpose of its great di- 

 ameter is the reduction of capillary depres- 

 sion in its mercury column. But its size 

 necessitates a very close approach of the 

 mercury to its upper end, in order to re- 

 duce suflScieutlj' its capacity. Yet the re- 

 maining space must be measurable by the 

 cathetometer, with the utmost precision. 

 Hence the glass must not be distorted by 

 heating, and the closed end just over the 

 mercury must be sharply defined. In con- 

 structing this part of the apparatus, I se- 

 lected a piece of heavy tubing which would 

 just slip inside of B, with the least possible 

 clearance. One end of this tube was closed 



as squarely as possible by fusion, and then 

 ground with fine emery and a suitable tool, 

 to a convex spherical surface of a long 

 radius. Care was taken to make the cen- 

 ter of curvature lie in the axis of the tube, 

 and the ground surface was left unpolished 

 to facilitate observation. A suitable length 

 of the closed end of the tube was then cut 

 off, slipped into B, and both tubes were 

 fused together at their open ends, as shown. 



For calibrating the head B, a ground 

 glass stopper with a capillary duct was 

 fitted to its neck, before the latter was sealed 

 to the bulb A. The head was then filled 

 with mercury by boiling, thus completely 

 filling the small space between its wall and 

 the cap. After cooling, the stopper was in- 

 serted to expel all excess of mercury, and 

 the whole weighed. l^Text the head was 

 emptied, and the mercury in the annular 

 space distilled out. Again the head was 

 very nearly filled with mercury, without al- 

 lowing any to get into the annular space, 

 and weighed as before ; and the space be- 

 tween the top of the mercury and the con- 

 vex end of the head was very- carefully 

 measured by the cathetometer. This pro- 

 cess of weighing and pleasuring was re- 

 peated several times, with less mercury 

 each time. Thus the capacity of a vertical 

 millimeter of the head was ascertained, as 

 well as the capacity that would remain, if 

 the top of the meniscus of mercury just 

 touched the convex end of the gauge, above 

 it. Finally the neck was sealed to the bulb 

 A, and the capacity of head, neck and bulb 

 combined was found by weighing them 

 empty, and again filled with mercury. 



For lighting the top of each mercury col- 

 emn, a narrow horizontal slit in an opaque 

 screen, R is used. The slit is covered with 

 a strip of ground glass and obliquely illu- 

 minated bj' an electric lamp. The screen 

 and slit are vertically adjusted by a thumb 

 screw S. The heat of the lamp is pre- 

 vented from reaching the mercury col- 



