LAND MAGNETIC OBSERVATIONS, 1914-1920. 

 EXPLANATORY REMARKS. 



Precisely the same conventions have been followed in the presentation of the 

 field results obtained during the seven years 1914 to 1920 as adopted in Volumes 

 I and II. These conventions, briefly recapitulated, are as given in the following 

 paragraphs. 



It has not been deemed advisable to attempt at present to apply corrections 

 to the observed results on account of the numerous variations of the Earth's mag- 

 netism, e. g., diurnal variation, secular variation, magnetic perturbations, etc. 

 Instead, it is believed to be better to publish the observed results as obtained, 

 with no corrections applied except the reductions to the magnetic standards of 

 the Department, as fully explained in the section on this subject; thus undue delay 

 is avoided in the promulgation of the results. The reduction to a common epoch 

 can be undertaken more advantageously later. It will be noticed, however, that 

 opposite the magnetic elements appearing in the Table of Results, the precise date 

 and local mean time of each observation are given. The reader is thus supplied 

 with the required information in case he may find it necessary to reduce the 

 observed values to some mean time. 



The following main geographic divisions have been adopted: Africa, Asia, 

 Australasia, Europe, North America, South America, Islands Atlantic Ocean, 

 Islands Indian Ocean, and Islands Pacific Ocean. Under each main division 

 there are broad subdivisions (see Africa, for example). The tabular entries under 

 these subdivisions are in the order of decreasing north or increasing south latitude; 

 that is to say, in the order of increasing colatitude counting from the North Pole 

 to the South. When there are stations of the same latitude, their order is accord- 

 ing to increasing east longitude, counting continuously from the standard meridian 

 of Greenwich, or from zero to 360 degrees. 



The question whether to give values of the horizontal intensity, exclusively, 

 or values of total intensity, was decided, for practical reasons, in favor of the 

 former. In the vast majority of cases, the horizontal intensitj^ rather than the 

 total is observed, and most likely will continue to be for some years at least. Only 

 in high magnetic latitudes, where the horizontal intensity is small and hence its 

 observation more or less difficult, are total intensities generally obtained. Rather 

 than give total intensities, as derived by computation with the aid of the observed 

 horizontal intensity and inclination, it was thought a better procedure to com- 

 pute, in the considerably smaller number of cases, the horizontal intensity from 

 the observed total-intensity and inclination, the so-obtained values being italicized 

 in order to reveal then derivation. 



It was also decided to publish the intensities in C.G.S. units. 1 In magnetic- 

 survey work on land the fourth decimal is often uncertain by one or more units and 



1 The capital gamma, r, was used in Volumes I and II to designate a C.G.S. unit of magnetic intensity; but as it is 

 not generally used for this purpose, its use by us was discontinued beginning with Volume III. L. A. B. 



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