10 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



CHAPTER II. 



THE NATURE AND INTERPRETATION OF THE GEODETIC 



EVIDENCE. 



Before dealing with the observations it will be well to devote 

 some space to a consideration of the nature of the evidence and the 

 bearing of the observations, when converted from their original 

 object, of measuring the dimensions and form of the earth, to that 

 of elucidating the structure of the outer portion which is called, 

 for convenience and brevity, its crust. Though the treatment 

 may be regarded as elementary by a portion of those who will 

 read these pages, it is none the less necessary for two reasons, firstly 

 because many may be unfamiliar with the nature and the meaning 

 of the observations, and secondly because, for those who may be 

 familiar with this aspect of the geodetic results, it is important 

 to have a clear understanding of the possibility, and more especi- 

 ally of the limitations, of their application to the completion or 

 checking of the results of geological observation, and it is this 

 aspect of them which will alone be dealt with. 



The geodetic observations which have to be considered may 

 be described as measures of the direction and intensity of the force 

 of gravity, and are of two classes. One deals with the deflection 

 of the plumb-line from the direction which it would have on the 

 surface of an ideal earth of perfectly regular form and uniform dis- 

 tribution of density, the other measures the variations in the attrac- 

 tion of gravity. Of these the first gives the horizontal and the 

 latter the vertical component of the resultant of all the forces 

 which produce a departure from the attraction which would be 

 exerted by the ideal average globe. 



The position of two places on the surface of the earth, with regard 

 to each other, may be expressed in two ways, either by a differ- 

 ence in longitude and latitude, or by the length and direction of 

 the shortest line connecting them. The determination of the first 

 of these belongs to the methods of astronomy, the latter to those 

 of trigonometrical survey, and the one could be converted into 

 the other with equal accuracy if we knew with absolute accuracy 



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