THE NATURE AND INTERPRETATION OE GEODETTC EVIDENCE. 23 



the absolute values be determined with certainty, but the differ- 

 ences, between the observed values in the stations taken into con- 

 sideration, may be depended on, and in both cases we have depar- 

 tures from the values reckoned, without consideration for the 

 disturbances produced by local departures from a condition of 

 uniformity, which are far in excess of any possible error in the 

 factors used in the calculations. 



The reduction of the observed value of the force of gravity 

 to sea level is a matter introducing much larger possibilities of 

 variation than the determination of the theoretical value at sea 

 level. The corrections to be applied are as follows : — 



(1) for the height of the station above sea level ; 



(2) for the attraction of the masses above sea level, but 



below the level of the station ; 



(3) for the attraction of masses which rise above the level 



of the station ; 



(4) for the effect of compensation of the elevated masses. 



The first of these depends on the fact that the force of gravity 

 decreases as the surface of the earth is left below. This correc- 

 tion can be applied with great exactitude and there is no doubt 

 of the reality of its effect ; it is sometimes referred to as the ' ; free 

 air" correction, as, in applying it, the whole of the underlying 

 ground is supposed to be removed and the station left standing 

 free in the air. Here it will be referred to as the correction for 

 height. 



The second correction is sometimes also called the Bottguei 

 correction, a term which refers to the particular method of calculation 

 adopted, by which the station is supposed to be situated on the 

 surface of a level plateau. The combined effect of this and the cor- 

 rection for the actual irregularity of the surface, often referred 

 to as the orographical correction, will here be referred to as the 

 correction for visible mass, that is for the attraction of all the mass 

 which lies above sea level at and around the station. 



The fourth correction is a modern development, first applied 

 by Messrs. Hay ford and Bowie, 1 and a consideration of its effect 

 is necessary for the understanding of the interpretation of the 

 anomalies discussed further on. 



1 The Effeet of Topography and Iso>tatic Compensation upon the Intensity of 

 Gravity, by J. F. HayfcnJ & W. Bow fe. Washington. 1912. 



[ 171 ] 



