GRAVITATIONAL METHODS 391 



survey. If the heat is turned off during long moves or discontinuance of 

 work, a sufficient time (usually 24 hours) is allowed for the instrument 

 to stabilize after it is again heated prior to new field work. 



In other meters, sufficient thermal insulation is provided to make the 

 temperature changes gradual, and then their effects are corrected for by 

 the drift curve. One make of gravity meter is placed in a large Dewar 

 flask or thermos bottle which serves as an excellent heat barrier. 



Leveling. — Accurate levels are provided so that when a gravimeter is 

 set up on its tripod or base plate it can be carefully leveled. The matter of 

 tripods is covered in more detail in the section on field work. 



Orientation. — Accurate orientation of a gravimeter is not ordinarily 

 necessary, at least to the extent that a special operation is required for that 

 purpose, as with the magnetometer. In routine field station set-ups the 

 gravimeter is placed on its tripod with approximately the same orientation 

 at each station. 



With certain older-type instruments, a slightly different reading was 

 obtained depending on the direction (north, east, south or west) in which 

 the field truck was headed when a reading was being taken. In such a case 

 the amount of the correction, which is only a scale point or two, is found 

 by trial and applied when calculating the field notes. 



Latitude Adjustment. — As has been noted, the normal value of gravity 

 force varies uniformly with latitude. From the equator to the poles, the 

 total gravitational change due to latitude is about 5.2 gals ; (see Table 6, 

 p. 254.) For a gravimeter with a scale value or scale constant of a frac- 

 tion of a milligal per scale point, this change would represent a very large 

 range in scale reading. 



To provide for this change, gravimeters have a means of adjustment 

 so that the scale is in view for work in a given area. If operations are 

 shifted to another area at appreciably different latitude, for instance between 

 South Texas and Colorado, the scale must be re-set for the new locality. 

 This adjustment is similar to the latitude adjustment to bring the scale 

 of a magnetometer in view when moving from one area to another. 



Base Station Gravity Value. — The values of gravity at the various 

 field stations shown on the final maps are relative to the main base station 

 of the survey. An arbitrary value in milligals is assigned to this base 

 station. This value should be selected at a convenient figure, usually large 

 enough so there will not be negative values in the gravity contours 

 (isogams). 



The assuming of an arbitrary base station value in gravity work is 

 comparable to using an assumed value of magnetic intensity at the mag- 

 netic base station. If desired, gravity data can be tied into pendulum 

 gravity stations, just as magnetic data can be tied into total values of 

 magnetic force at magnetic bench marks. This may be accomplished by 



