METEOROLOGICAL INSTRUMENTS. 217 



rise, and vice versa ; and as the height of the column to be mea- 

 sured is that above the level of the cistern, it will be impossible to 

 measure, this correctly by a fixed scale. This difficulty does not 

 arise in syphon barometers, as in them the mercury, when it leaves 

 the long leg, passes into the short leg, and vice versa; and the 

 height measured is the difference in length of the columns in the 

 two legs. 



1. In ordinary barometers with closed cisterns a certain height 

 of the column is correct by the scale ; this is called the neutral 

 point. The proportion between the sectional areas of the tube 

 and cistern is calculated; and from these data it is possible to 

 calculate what is called a " capacity correction," which is applied 

 to the reading on the fixed scale according as the level is above 

 or below the neutral point. 



2. In Fortin's Barometers, the construction of which is usually 

 adopted for standard barometers in this country, and several 

 specimens of which are exhibited, the scale is fixed, and its 

 lower end, or zero, is visible in the cistern, and is formed by 

 an ivory point. The level of the mercury in the- cistern must 

 be raised or lowered, so as to coincide with this point, by 

 means of a thumb-screw, which is attached to the base of the 

 cistern. This base is made of leather or other flexible material, 

 which is movable by the action of the screw, but yet is practically 

 impermeable to mercury. 



It is evident that this construction allows of a direct measure- 

 ment of the height of the column. 



3. In Marine Barometers i.e. in the Kew Marine Barometer, 

 devised by P. Adie in 1854, and recommended by the Kew Com- 

 mittee of the British Association to the Board of Trade in the 

 same year a totally new principle of construction has been intro- 

 duced. The extreme length of the scale is marked on the instru- 

 ment, but, instead of laying off true inches, the inches are shortened 

 from the upper part downward in proportion to the relative size 

 of the diameters of the tube and cistern respectively. 



