GRAVITATIONAL METHODS 



413 



GEAVITT-HKTER SURTET 



CORBECTION SHEET 



Oravlmeter No. 2 / 



Date v/^.^/ 2 3, /f4Z. 

 Computer /}■ tV. 



pressure or buoyancy correction is listed in column 7. The amount of the 

 correction for a given air pressure is obtained from a table which has 

 been previously vi^orked out by tests on the particular instrument used. 

 When the meter is not housed in a hermetically sealed case, variations in 

 air pressure are important. 



The instrumental corrections are added and applied to the reading of 

 column 3, and the result recorded in column 8, which gives the corrected 

 reading at the station in scale points. 



The base graph value of column 9 represents the reading that would 

 have been obtained at the base at a given time. It is found from the base 

 graph, or drift curve, for the time at which the station was taken. 



The base graph is plotted from base station check readings as shown 

 in Figure 245 and gives the instrument reading, at the base, versus time. 

 Instrument drift with a gravimeter, as noted, is purely an instrumental 

 phenomenon, as there is no detectable daily variation in gravity force. A 

 tidal variation of gravity, relating to the position of the sun and the moon, 

 is just slightly below the range of operation accuracy of gravimeters and 

 usually is neglected. 



Column 11 gives the difference in reading at the station relative to the 

 base and is obtained by subtracting the base graph value from the corrected 

 station reading (column 8 minus column 9). 



The instrument constant k shown in column 1 1 is applied to the gravity 

 differences, in scale points, of column 10, and the result or gravity differ- 



