2i G SCIENTIFIC APPARATUS. 



PRESSURE. 



The instrument by which this is measured is termed a Baro- 

 meter ("weight measurer"), a name said to have been intro- 

 duced by Boyle. The instrument was invented by Torricelli, in 

 1643. In its very simplest form it consists of a tube of glass, 

 about thirty-four inches in length, closed at one end, filled with 

 mercury, and placed with the open end dipping into a cup con- 

 taining mercury, called the " cistern." The column of mercury 

 stands in London at a height varying from about 28 to nearly 31 

 inches, according to the weather, &c., above the level of the 

 mercury in the cistern. The standard barometer at Kew Observa- 

 tory is an instrument of this very elementary construction. The 

 height of the column is measured off by applying a rule to the 

 side of the tube, and for scientific purposes the height is read off 

 by an instrument called a Cathetometer, which consists of a tele- 

 scope kept in a horizontal position and movable along a stiff 

 vertical bar, which is carefully graduated. The telescope is suc- 

 cessively directed to the top and bottom of the column, and care- 

 fully adjusted by micrometer screws. The difference of the 

 readings gives the height of the column. 



For ordinary purposes there are two great classes of barometers. 

 In this we refer only to mercurial barometers, and omit all special 

 notice of barometers mounted in wooden frames, &c., inasmuch 

 as such instruments are not fitted for scientific observations, as 

 explained in the " Instructions in the use of Meteorological Instru- 

 ments," p. 12. 



These classes are 



A, Cistern Barometers ; B, Syphon Barometers. 



A. In cistern barometers there is one great difficulty to be pro- 

 vided against, arising from the fact that there is a definite quantity 

 of mercury in the instrument, and that when the level of the 

 column sinks in the tube, that of the liquid in the cistern must 



