PRESIDENTIAL ADDRESS. 753 



clo so; and in the case of the African arc this lattei? course is fottunately 

 possible. 



If we take the map of Africa we shall see that the are along the thirtieth 

 meridian passes through, or near, all the colonii's of British South Africa, close to 

 British Central Africa, or Nyasaland, through Uganda, and is thus connected 

 with British East Africa, through the British Soudan and through Egypt. There 

 remain absolutely untouched by it only the AVest Africa colonies — Nigeria, the 

 Gold Coast, Sierra Leone, and the Gambia. These latter will eventually get their 

 geodetic framework by an extension southwards of the French triangulation of 

 Algeria, a work of a high order of precision. We are therefore entitled to say — ■ 

 and I take this opportunity of sayiug it with all due emphasis — that with the 

 exception of some triangulation to joiu the West African colonies with the French 

 triangulation, the arc along the thirtieth meridian is the only primary triangula- 

 tion required for the adequate mapping of the whole of British Africa. The 

 remainder of the geodetic framework can be supplied by ribs of secondary 

 triangulation branching out from the main backbone, such as the line already 

 completed along the boundary between British and German East Africa, passing 

 to the north of the Victoria Nyanza and thence westward to the thirtieth 

 meridian. 



You Avill observe that I here speak only of the triangulation required for 

 mapping purposes, not of that demanded by the geodesist for the study of the 

 figure of the earth. The latter is satisfied only with a survey of the highest 

 attainable precision covering as large an area of the earth's surface as possible, or 

 at all events with arcs, both meridional and longitudinal at frequent intervals. It 

 cannot be other than a very long period before the whole of Africa is surveyed 

 upon this scale of accuracy, and in the meantime we must devote ourselves to the 

 far more urgent duty of mapping the country, leaving the more remote and 

 abstract task to our descendants, well satisfied if in our hands the foundations 

 have been well and truly laid. 



Furthermore, as we shall see presently, if we are prepared to recognise as a 

 national duty the minutely precise survey of our own land and of all territories 

 under our flag— and I do not see how any reasonable man can withhold this 

 recognition — then there are duties of this nature lying closer to our hands than 

 any to be found in Africa. 



Having thus passed in brief review the ultimate geodetic basis of our African 

 surveys, let us enter more into detail and glance at the actual survey work now 

 in progress in the different regions of the continent. 



In British South Africa, as we have already noted, the political conditions 

 are at present unfavourable to any comprehensive scheme of operations. There is 

 however in progress a first-class topographical survey of the Orange River 

 Colony and a reconnaissance survey of Cape Colony. The former is an excellent 

 example of the class of work that can be done by a small military party of the 

 highest technical training working upon systematic lines, and T should like to 

 devote a few minutes to a short description of the methods adopted and of the 

 results obtained. 



The survey party consists of two Royal Engineer officers and four non- 

 commissioned officers, the former undertaking the triangulation and the general 

 supervision of the field work, and the latter the plane tabling. The positions 

 are primarily hased upon the points of the geodetic survey broken up into ii 

 secondary triangulation with sides averaging ten miles. In 1907 *he average 

 triangular error of the secondary work was 2d seconds of arc, and the greatest linear 

 errors of displacement, as tested by the geodetic triangulation at the end of a 

 chain forty-five miles long, were three feet in latitude and two feet in longitude. 

 The probable error of a trigonometrical height was under a foot. You will see 

 therefore that the accuracy is ample for all mapping purposes, even upon large 

 scales, and the degree of precision is in excess of that demanded for j, topogra- 

 phical map on the scale of two miles to an inch. The rate of progress and the 

 low cost of work are, however, no less notable than its accuracy. The actual rate 

 of outturn is about eight square miles per day per man, or for the whole party 



1908. 3c 



