CONSTRUCTION OF A STANDARD BAROMETER. 443 



erected. The top of the column presents a good convexity in all 

 states of the barometer, with only a very slight trace of dirt. No 

 appearance of air-specks can be detected, except a few very minute 

 ones near the lower end of the tube, which have existed since the 

 commencement, and were produced by the temporary entanglement 

 of a small air-bubble at the shoulder-bent part of the syphon tube in 

 the operation of filling. These specks have not increased in number 

 nor shown any tendency to rise. A portion of the syphon being 

 retained at the lower end of the tube, it is highly improbable that 

 any air can now enter, the mouth of the syphon being cut off from 

 communication with the external air by the mercury in the cistern. 

 The tube extends to about nine inches above the mean height of the 

 mercury. 



The tube is supported over a glass cistern in a strong brass frame 

 secured by brackets to the wall of the old mural quadrant of the 

 observatory, the height of the mercury being measured by a catheto- 

 meter* fixed to the same wall at a distance of five feet. A conical 

 point, at the lower end of a short rod of steel, is adjusted by a screw 

 to the surface of the mercury in the cistern. At the upper end of 

 .the steel rod, and above the level of the glass cistern, is a fine mark, 

 whose distance from the conical point has been found by comparison 

 with the Kew standard scale to be 3.515 inches. When an observa- 

 tion is made, the lower point is adjusted to exact contact with the 

 mercury in the cistern; the telescope of the cathetometer is then 

 levelled, and its horizontal wire made to bisect the mark on the 

 upper end of the steel rod, the scale reading of the cathetometer 

 being noted. The telescope is then raised, again levelled, and the 

 wire made a tangent to the surface of the mercury in the tube, the 

 cathetometer scale reading being again observed. The difference be- 

 tween the two readings of the cathetometer scale added to the length 

 of the steel rod is the height of the column of mercury. Besides the 

 rod terminating in the conical point, a second adjusting rod is provided, 

 whose lower extremity is a straight edge. No difference could be de- 

 tected between the results from the two methods of adjustment. In 

 order to avoid the inconvenience of light being reflected into the tele- 

 scope from the surface of the mercury in the tube, a movable screen 

 is provided, the upper part of which is black and the lower part oiled 

 paper, which is so adjusted as to shut off all light which comes from 

 a higher level than the top -of the mercurial column. The surface of 

 the mercury thus presents in the telescope a well-defined dark out- 

 line. A window behind the barometer gives a good illumination to 

 the paper screen; a lamp being required at night. A thermometer 

 whose bulb is within the mercury of the cistern gives its temperature, 

 and the scale of the cathetometer being of brass, the usual tables can 

 be employed for the temperature correction, the difference in the 

 expansion of steel and brass being insignificant for the length of the 

 short adjusting rod. The variations of the temperature of the room 



* A small telescope, with a horizontal wire in the focus of the eye-piece, sliding on a 

 vertical graduated measuring rod. 



