BAROMETER, ETC. AT THE KEW OBSERVATORY. 
509 
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 9 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 Cathetometer fixed to the same wall, at a distance of 
5 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 (see Plate XX. fig. 2). 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 observation is made, the lower point 
is adjusted to exaet contact with the mereury 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 
teleseope 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 between 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 detected between the results 
from the two methods of adjustment. In order to avoid the inconvenience of light 
being reflected into the telescope from the surface of the mercury in the tube, a 
moveable 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 outline. A window behind the barometer gives 
a good illumination to the paper screen, a lamp being required at night. A thermo- 
meter 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 tempe- 
rature of the room are not rapid, so that no sensible error arises from assuming the 
temperature of the Cathetometer to be the same as that of the mercury. The cistern 
of the Standard Barometer is 33*9 feet above the mean level of the sea, being 9T feet 
above the Ordnance bench-mark on the north-east corner of the Observatory, whose 
elevation is stated by Lieut.-Colonel James to be 24'83 feet. 
Observations of this barometer being too troublesome when an extensive series is 
required, a Standard, by Newman, No. 34, having a tube of 0‘55 inch, which has been 
repeatedly compared with the great Kew Standard, is employed for ordinary use ; 
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