PREFACE. 



In the preparation of the following work two difficulties of quite different 

 kinds presented themselves. The first of these was to make a judicious selec- 

 tion of matter suited to the needs of the average geographer, and at the same 

 time to keep the volume within prescribed limits. Of the vast amount of 

 material available, much must be omitted from any work of limited dimen- 

 sions, and it was essential to adopt some rule of discrimination. The rule 

 adopted and adhered to, so far as practicable, was to incorporate little material 

 already accessible in good form elsewhere. Accordingly, while numerous ref- 

 erences are made in the volume to such accessible material, an attempt has 

 been made wherever feasible to introduce new matter, or matter not hitherto 

 generally available. 



The second difficulty arose from the present uncertainty in the relation of the 

 British and metric units of length, or rather from the absence of any generally 

 adopted ratio of the British yard to the metre. The dimensions of the earth 

 adopted for the tables are those of General Clarke, published in 1866, and now 

 most commonly used in geodesy. These dimensions are expressed in English 

 feet, and in order to convert them into metres it is necessary to adopt a ratio of 

 the foot to the metre. The ratio used by General Clarke, and hitherto gener- 

 ally used, is now known to be erroneous by about one one hundred thousandth 

 part. The ratio used in this volume is that adopted provisionally by the Office 

 of Standard Weights and Measures of the United States and legalized by Act 

 of Congress in 1866. But inasmuch as a precise determination of this ratio is 

 now in progress under the auspices of the International Bureau of Weights and 

 Measures, and inasmuch as the value for the ratio found by this Bureau will 

 doubtless be generally adopted, it has been thought best in the present edition 

 to restrict quantities expressed in metric measures to limits which will require 

 no change from the uncertainty in question. In conformity with this decision 

 the dimensions of the earth are given in feet only, and, with a few unimportant 

 exceptions, to which attention is called in the proper places, tables giving quan- 

 tities in metres are limited to such a number of figures as are definitely known. 



