BAROMETER, ETC, AT THE KEW OBSERVATORY. 
511 
siderably different pressures with a standard instrument, that is, with one in which 
the mercury is adjustable at each observation to a constant zero-point. This is done 
by placing- the barometer and a standard within a receiver provided with the means 
of altering at pleasure the pressure of the enclosed air. 
The Receiver is of cast iron, its horizontal section being rectangular (Plate XX. 
figs. 3 & 4j: it is 39 inches high, 12 inches by 6^ at its lower end, tapering to 10 
inches by 4^ at the upper end; there being room for three marine barometers besides 
the Standard. Windows of strong plate glass, each 11| inches high and 9^ inches 
wide, let into both sides of the receiver, admit of the barometers being observed by 
a Cathetometer. Smaller windows below, each 3 inches square, show the cistern of 
the Standard Barometer, the mercury in which is adjusted to a constant level by a 
screw passing through a stuffing-box in the base of the receiver. The barometers 
to be verified are suspended by a girnbal arrangement from the upper end of the 
receiver, a massive lid closing the opening at the top by which they are introduced. 
An opening in the base, furnished with a stopcock, is connected by a flexible tube 
with a pump which regulates the pressure of the enclosed air. This pump consists 
of a single barrel and piston : there being openings at both ends of the barrel, the 
valves are so arranged that, when the flexible tube is attached to the lower opening, 
air is extracted from the receiver, and when with the upper, air is forced in. The 
receiver is supported by an iron bracket, securely fixed to the quadrant wall about 
10 feet from the great Standard Barometer. The Cathetometer, being between the 
receiver and the Standard Barometer, can be used at pleasure for either. The adjust- 
able barometer used in the receiver for comparison with the marine barometers, has a 
tube 0'35 in diameter, there being a contraction in the tube of the same kind and to 
about the same degree as in the ordinary marine barometers made by Mr. Adie. 
This apparatus for the verification of marine barometers has (with the exception of 
the adjustable barometer, which is by Mr. Adie) been entirely constructed in the 
Observatory by Mr. Robert Beckley the Mechanical Assistant, who has executed the 
work in a most satisfactory manner, and who has shown much ingenuity in arranging 
the mechanical details, so as to afford the utmost exactness in observation and con- 
venience in manipulation. 
The mode of observation is the following: — Supposing air to have been extracted 
from the receiver until the barometers stand at about 27 inches, sufficient time having 
elapsed to allow the mercury to come to a state of rest, and the zero of the Standard 
having been adjusted, the height of the mercury in each of the barometers is observed 
by the Cathetometer. Air is then admitted until the mercury stands at about 31 
inches, when the same operation is repeated. The length of the graduated scale of 
the barometer under comparison is then measured by the Cathetometer. If A, a 
be the Cathetometer readings, at the higher pressure, of the Standard and marine 
barometers repectively ; B, h those for the lower pressure ; and if L be the measured 
length of one inch of the scale of the marine barometer, then the Correction for Capa- 
