200 



TV. MAEKIOTT — EEDXJCTION 0¥ 



(3). The coiTOction for hcipjht above sea-level varies not only with 

 the altitude, but also Avith the temperature and pressure of the air. 

 Table II. gives the corrections for reducing the barometer readings 

 to sea-level for every 20 feet up to 500 feet, for the air temperatures 

 of 20°, 40°, 60°, and 80°, at the sea-level 2}ressures of 28-0 ins., 

 29-0 ins., and 30-0 ins. From this table should be prepared 

 another one for the height of the particular station for which it is 

 to be used, giving the corrections for altitude for every tenth of an 



Table III. — For Reducing Ohservations of the Barometer to Sea-level. 

 STATION 240 FEET. 



inch pressure from 28'5 ins. to 30'6 ins., and each 10° of tempera- 

 ture from 20° to 80°. Table III. is a specimen of this form for 

 240 feet, the height of "Watford. This table is prepared in the 

 following manner : the form having been ruled, and the tempera- 

 tures at the top and tlie sea-level pressures in the left-hand column 

 having been filled in, the corrections for 240 feet, at 29-0 ins. and 

 30-0 ins., are copied from Table II., and entered on the lines 

 representing these values ; the intermediate readings are then filled 

 up by interpolation. When this has been done, it will be seen 



