1858.] and the Figure of the Earth. 203 



are rising, and that this alternate action has been going on for ages. 

 The huge mass appears like a gigantic monster heaving its ribs and 

 then drawing them in again, but with a deliberation which can be 

 measured only by something like astronomical or rather geological 

 periods, and through spaces, though minute, yet sufficiently sensible 

 to destroy the symmetry of its form. 



4. — It is therefore perfectly gratuitous to assume as has generally 

 been done, that the form of the earth is now an exact spheroid. 

 And when we look back to the methods which have been used with 

 such success to determine the degree of oblateness, it will be 

 observed that they all of them regard the earth as a whole, and 

 take no account of its separate parts. Indeed, as I have already 

 intimated, it lias long been an acknowledged fact, that the 4th 

 method fails when arcs near each other, and therefore appertaining 

 to any one portion of the earth's surface alone, are compared ; and, 

 I believe myself, chiefly for this reason, that the earth's form is not 

 an exact spheroid. The spheroid (of depression l-300bh) which 

 has been determined by these four methods is, therefore, the average 

 spheroid ; or the spheroid which more nearly represents the earth's 

 irregular form than any other spheroid ; some parts being slightly 

 above it, and some slightly below it, owing to the irregularities 

 which have arisen since the earth ceased to be a fluid mass. We 

 can no longer assume that the arcs of meridian are all equal ellipses, 

 or are ellipses at all, or that the arcs of longitude are circular. 



Remarks on Mr. Tennanfs Papers. 



5. — To ascertain the actual form of the different parts of the 

 surface, each part must undergo a separate examination ; as the 

 form, though nearly spheroidal, is not exactly so and follows no 

 known law. When we wish to measure the curvature of a curve 

 not differing much from a circle, it is convenient to compare it with 

 the ellipse which most nearly approaches it in form, as the ellipse 

 is the next simplest curve to the circle. Any arc of meridian drawn 

 upon the surface of the earth departs but little from a circle, and 

 may therefore be thus compared. In this comparison, for conve- 

 nience' sake, the ellipse is so chosen as to have its centre in the 

 centre of the earth and one of its axes coincident with the earth's 



2 J) 2 



