ANNOTATIONS AND ADDITIONS. 157 



means of a particular method of shading, what is still 

 unknown, and what, by the courage and perseverance of 

 travellers of all nations, among whom our own countrymen 

 happily hold an important place, has been already 

 disclosed to us. It is a valuable service, and one which 

 opens the way for farther advances and more comprehensive 

 inferences, when persons,' thoroughly acquainted with the 

 existing, often widely scattered, materials, men who do not 

 merely draw and compile, but compare, select, and, where- 

 ever it is possible, check and control the routes of travellers 

 by astronomical determinations of position, undertake to 

 represent graphically the results of the elements of knowledge 

 possessed at the time. Those who have themselves given 

 to the world so much as you have done, have an especial 

 right to expect much ; since their combinations have largely 

 augmented the number of connecting points ; yet I believe 

 that when you executed your great work on Africa in 1822 

 you could hardly have expected so many accessions as we 

 have now received." The knowledge acquired is, indeed, 

 often only that of rivers, their direction, their branches, 

 and the various synonyms by which they are called in 

 dialects belonging to different families of languages; but 

 rivers reveal to us by their course the form of the surface 

 of the earth, and are at once the nourishers of vegetation, 

 the channels of intercourse between .men, and pregnant with 

 unknown influences on the future. 



The northerly course of the White Nile, and the south- 

 easterly course of the great Goschop, would indicate that a 



