154 * EXPLORATION GEOPHYSICS 



called North and East correction). Station 1 is 1 mile N. and 2 miles E. 

 This means 11.8 + (2 x 4.7) = +21.2 y's; use 21 y's. Station 2 is 1 mile 

 N. and 1 mile E., or 11.8+ 4.7 = 16.5 ; use 17 y's. Station 3 is 1 mile S. or 

 —11.8 y's; use —12 y's. 



It is a feature of this form that corrections are entered with their 

 true signs. That is, if the daily variation decreases or is less, its sign in 

 the form is — . Likewise, if the L. and L. correction represents an increase 

 of the earth's field by virtue of a station being north of the base, the correc- 

 tion goes into the form as +. The correction is applied in the proper 

 manner as a result of the operation of the form, as will be explained later. 



In column 12 is entered the reading in y's obtained at the base when 

 first occupied ; in this case it is 572, which is recorded in this column after 

 each station and applied as a correction. It is subtracted from the reading 

 at each station in the operation indicated in column 14. 



In column 13, the corrections of columns 9, 10, 11, and 12 are added 

 and set down as shown. Column 14 represents the operation column 8-13, 

 that is, the station reading minus the sum of the corrections. The values 

 for the stations, as shown in column 14, are the vertical intensity values 

 with all corrections applied, assuming the base has a value of zero. They 

 indicate how much higher or lower a station is, magnetically, than the 

 base. 



The diurnal and L. and L. corrections are properly applied in the oper- 

 ation of this form. Consider only the daily variation. This variation shows a 

 decrease of 3 y's for station 1. To eliminate this change and get the 

 reading at station 1 on a basis of no daily variation, we would have to 

 add to the reading at station 1 the amount of 3 gammas. This is actually 

 what has been done, because in the operation of column 14 (considering 

 the daily variation alone) the base value 572 was to be subtracted from 

 the station value 622. We have subtracted from 622 a number three less 

 than 572, or in effect we have added 3 gammas. 



In like manner, considering only the L. and L. correction at station 1 

 of 21 y's, the earth's field has increased 21 gammas at this station. To 

 apply this correction properly and put station 1 on the same magnetic 

 datum as the base, 21 gammas must be subtracted from the reading at 

 station 1. This is done because we subtract from the 622 value for station 

 1 the base value 572 + 21 gammas. Considering L. and L. correction only, 

 we take from 622 a higher number than 572, or 593. 



To follow this through on another line : take the station reading for 

 station 1 of 622 and add to it the 3 y's of daily variation, which makes 625. 

 From 625 subtract the 21 y's of L. and L. change, leaving 604. From 604 

 take the base reading of 572 which leaves +32, the value previously 

 obtained for station 1. 



In column 15 the arbitrarily assumed value (X) assigned to the base 

 is added to the values of vertical intensity obtained in column 14 relative 

 to the base as zero. This value X is taken at a convenient figure for the 



