CONSTRUCTION OF A STANDARD BAROMETER. 445 



becomes therefore necessary to determine the correction for "capa- 

 city," or the variation in the zero point corresponding to different 

 heights of the column of mercury. The amount of this correction 

 may be determined during the construction of the instrument; or, by 

 reducing in the required proportion the lengths of the divisions, it 

 may be allowed for in graduating the scale, as has been done in the 

 marine barometers made under the supervision of the Kew committee 

 by Mr. P. Adie, of London. In order to test the accuracy of this 

 correction, it is necessary to compare the barometer at two consider- 

 able 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 inclosed air. 



The receiver is of cast iron, its horizontal section being rectan- 

 gular. It is 39 inches high, 12 inches by G.^ at the lower end, and 

 tapering to 10 inches by 4^ at its 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 three 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 gimbal 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 stop-cock, 

 is connected by a flexible tube with a pump which regulates the 

 pressure of the inclosed air. The 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 ittached 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, se- 

 curely fixed to the quadrant wall, about 10 feet from the standard baro- 

 meter. The cathetometer being between the receiver and standard 

 barometer, can be used at pleasure for either. The adjustable barome- 

 ter 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 ma- 

 rine barometers made by Mr. Adie. This apparatus for the verifica- 

 tion 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 convenience in mani- 

 pulation. 



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 



