LAND MAGNETIC OBSERVATIONS, 1911 1913. 

 EXPLANATORY REMARKS. 



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

 field results obtained during the three years 1911 to 1913 as adopted in Volume I. 

 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 

 magnetism, 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, probably after 1914, when additional 

 data have been secured. The reader will notice, however, that opposite the mag- 

 netic elements as tabulated in the Table of Results the precise date and local mean 

 time of each observation are given; he is thus supplied with the required informa- 

 tion in case, for some purpose of his own, it is 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, Islands Pacific Ocean, and Antarctic Regions. 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 co-latitude counting from 

 the North Pole to the South. When there are stations of the same latitude, their 

 order is according to increasing east longitude, counting continuously from the 

 standard meridian of Gi'eenwich or 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 intensity 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 compute, 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 their derivation. 



It was also decided to publish the intensities in C. G. S. units, one C. G. S. unit 

 being designated by capital gamma, F. In magnetic-survey work on land the fourth 

 decimal is often uncertain by one or more units and in ocean work the error may 

 be five or more units in this decimal place. For these reasons it appears inad- 

 visable for field results to adopt so small a unit as a small gamma, y = 10'^ 



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