I04 



NA TURE 



[June 2, 1904 



THE PRESENT POSITION OF GEODESY. 



"T^HE article by Commandant Bourgeois in the 

 -*■ Revue Ginerale des Sciences for April 30, on the 

 present position of geodetic science is both instructive 

 and useful with reference to those problems in geodesy 

 which are just now before the scientific public of this 

 country. There is, in the first place, a notable scheme 

 for the construction of a geodetic arc in Africa which 

 shall extend from the Cape to Cairo. Of this Com- 

 mandant Bourgeois has taken due note, entering 

 rather fully into the details of such difficulties as its 

 projectors may find in the way of its successful accom- 

 plishment. There is also an agitation recently 

 started _ amongst astronomers and surveyors, which 

 has for its object the revision of the geodetic triangula- 

 tion of England in order that it may be brought into 

 Hne,_ scientifically, with the geodetic triangulation of 

 adjoining countries, and take^its place (as it should) as 

 a link in more than one European system of which the 

 value would be largely increased 'bv this extension. 

 Of this Commandant 

 Bourgeois takes no 

 note (probably because 

 he is unaware of its 

 existence), nor does he 

 concern himself with 

 any past achievements 

 in the field of geodesy 

 in which England has 

 borne a part either at 

 home or in India. 



The object of the 

 article is to place be- 

 fore the reader the 

 effect of fresh inven- 

 tions and new methods 

 in developing existing 

 geodetic projects, but 

 it would have added 

 much to the interest 

 of it if so competent 

 an expert as Com- 

 mandant Bourgeois 

 had written something 

 about the change 

 which has come over 

 the objective of geo- 

 detic science which 

 justifies its continued 

 application to modern 1 1 > ) -i lu 



fields of surveying. 



These are utilitarian days, and seeing that the science of 

 geodesy long ago evolved all the necessary factors 

 for the reduction of astronomical and terrestrial observ- 

 ations by giving us certain mathematical formute based 

 on the measurement and form of the earth, and that 

 no subsequent investigations will ever seriously affect 

 those deductions, it may well be doubted if any State 

 financial assistance would be justifiable for the mere 

 purpose of refining and polishing the results of what 

 would be a purely abstract scientific inquiry. Geodetic 

 arcs measured simply for the purpose of ascertaining 

 the nature of certain eccentricities in the figiire of the 

 globe will no longer be regarded as worthy of the sav- 

 ing grace of State financial support, and it will remain 

 for their projectors to prove that some other and more 

 practical end is to be served by them if they wish for 

 substantial recognition. 



There is, of course, another (and an insufficiently 



appreciated) end to be served by such exact scientific 



processes as are involved in the measurement of a 



" great arc "; and if we drop the somewhat misleading 



NO. 1805, VOL. 70] 



term "geodetic," and simply appeal to the absolute 

 necessity for a strong initial- backbone of first-class 

 triangulation as the basis of every survey scheme of any 

 consequence at all — a backbone which will support the 

 weight of any subsequent superstructure of looser and 

 more rapid forms of triangulation which may be built 

 upon it, and thus give solidity and homogeneity to the 

 whole mapping of a vast area (such as Africa, for in- 

 stance), we only indicate the same thing under a far 

 more practical and intelligible form. All surveyors are 

 agreed as to the necessity for such an initial backbone, 

 although perhaps opinions may differ as to how far it 

 should be extended. The great value of Commandant 

 Bourgeois's article lies in this — that he shows clearly 

 and concisely how the best possible scientific results 

 may be obtained by means which not long ago were 

 unattainable, and which involve half the expense, with 

 (possibly) double the accuracy of those older methods 

 which cost the country so much in the past, and (in 

 the case of England, at least) have not proved satis- 

 factorv in the end. 



ipparatus [-laced in position for base measurement. 



The Jaderin apparatus for base measurements, and 

 the application of the French metal " invar " 

 (an amalgam of 64 steel to 36 nickel) to it, is 

 perhaps the most important of all recent improve- 

 ments in the process of constructing a primary, or 

 "first-class," triangulation. The old, clumsy, and 

 inconvenient method of measurement bv compensation 

 bars has, we hope, disappeared for ever. The 

 Jaderin tape has been exhaustively tested under 

 other conditions than those mentioned by Commandant 

 Bourgeois, and it has been found to stand the test of 

 extremes of climate quite sufficiently well. The author 

 indeed advocates its use for the measurement of long 

 bases in supersession of the system of extension by 

 triangulation from a short one. The rapidity and 

 accuracy with which this method can be applied to the 

 base measurements connected with a long series of 

 principal triangulation is instanced in the case of the 

 North American meridional arc, which has been mea- 

 sured on the 98th degree of west longitude. In this 

 instance nine bases were measured in one field-season 



