378 



Correspondence — Prof. A. H. Green. 



Let the dips observed along two lines AB, AC, Fig. 1, be 1 in m 

 and linn; make AB m units, AC n units in length ; join B C, and 

 draw A D perpendicular to B C. 



Then AD \% the direction of the full dip, and \i AD contain d units 

 of len^h, the full dip is 1 in d. 



A similar construction will give the apparent dip in any direction 

 when the full dip is known. Let AD he the direction and 1 in d 

 the amount of the full dip ; AB the direction in which the dip is 

 required. 



Make ADd units in length, draw DB perpendicular to AD meet- 

 ing AB in B. 



Then if AB contain x units of length, the dip along AB is 1 in x. 



If the two dips, instead of being both towards or both away from 

 A, be one towards and one away from A, produce one of the lines 

 A B or AC to E, Fig. 2, make AB, AE, m and n units in length, 

 draw ^ i^ perpendicular to BE, and A i^ will give the direction and 

 amount of the full dip as in the first case. 



For small angles the value of the dip expressed as one in so many 

 which corresponds to a given number of degrees, may be obtained 

 approximately by finding how often the number of degrees is con- 

 tained in 60. In the table below the first column gives the value 

 correct to two places of decimals for the angle opposite ; in the 

 second column are the values given by the above approximate rule ; 

 in the third the angle to the nearest minute that corresponds to the 

 values in the second; and in the fourth the error committed by 



