318' MM. Struve and Dollen on the Correction 



limits of the earth's body (we mean the geometric figure) still 

 retaining, as before, the truly spherical form. It may appear, 

 to say the least, inexpedient to use the term ' figure of the 

 earth ' in a sense so essentially different from that which has 

 the best claim to be so called. This objection however disap- 

 pears on closer enquiry. Such enquiry shows us that in pram 

 (that is, upon our earth as it is) this difference does not exist ; 

 or, to speak more correctly, does not exist for the greater part 

 of it, because it is covered, at least over a very large portion 

 of its surface, by a continuous fluid envelope. In this, the 

 fluid portion of the surface, which we call the ocean, supposed 

 at rest, the geodetic figure corresponds precisely with the 

 geometric figure; for it is in the nature of a fluid that its surface 

 must everywhere be perpendicular to the resultant of all the 

 forces acting at each point. Such coincidence is certainly 

 not necessary in the other, the solid, portion of the earth's 

 surface; yet, from the fact that the mean height of land above 

 sea, so far as our present knowledge extends, nowhere shows 

 a relation to geographical position, we infer that here also the 

 accordance of the geometric figure with the geodetic exists 

 as a matter of fact — a position in the physical theory of the 

 earth which, as is known, forms a starting-point for further 

 arguments as to the condition of the earth's crust. It follows, 

 therefore, that geodesy, which is of course limited, by the 

 nature of its operations, to the solid portion of the earth's 

 surface, is nevertheless in a position to determine the figure of 

 the whole earth. In any case — that is to say, even supposing 

 that in the future the figure of that solid portion should be re- 

 cognized as quite different from that of the ocean — these 

 geodetic results will still remain as an indispensable foundation 

 for all further research. 



" Our experience so far leaves us no longer any doubt that 

 the figure of the earth, strictly considered, is an altogether 

 irregular one. Should we now engage further in the at- 

 tempt (surely a fitting one for geodesy to undertake) to 

 know this irregular figure in all its particulars, then clearly we 

 cannot avoid comparing the linear determinations on its sur- 

 face individually with the corresponding angular celestial 

 measures, by means of which the amount of change of gravity- 

 direction becomes known. Any alteration whatsoever of one 

 or other of the quantities so obtained by direct observation, 

 would be plainly unreasonable. 



" The scientific craving, however, after knowledge of law, on 

 the one hand, and on the other, the practical need of a know- 

 ledge of the figure of the earth as a whole, as well as of those 

 parts where no direct measurements exist, impose upon 



