206 Mr. Ivory's Argtiments tending to prove that 



clusively to solids of revolution ; and thei'efore such, it would 

 seem, must be the figure of the earth, either exactly, or so 

 nearly that the difference is insensible to our observations. 



If, according to the foregoing reasoning, we assume that 

 the earth is a spheroid of revolution, and that the dimensions 

 of the meridians are known, we are in a condition to calculate 

 every circumstance relating to a line measured upon its sur- 

 face, provided we have ascertained the position of the extre- 

 mities of that line. From actual measurements made either 

 exactly perpendicular to the meridian, or nearly so, we can 

 deduce the length of a perpendicular degree at a certain lati- 

 tude on the given spheroid ; and if it be found that the result 

 thus obtained accords with the length of the same degree de- 

 rived solely from the dimensions of the meridians, there is a 

 presumption at least in favour of the assumed figure of the 

 earth. It was in this point of view that, in some former Num- 

 bers of this Journal, I examined some of the most remai'kable 

 measurements perpendicular to the meridian. The computa- 

 tions that were made, prove at least that the same compression 

 which represents very accurately the lengths of the meridional 

 arcs, agrees equally well with the pei'pendicular measurements. 

 It would, no doubt, have been very desirable and more satis- 

 factory, if we had had it in our power to compare the com- 

 puted difference of longitude of the extremities of the measured 

 line, with the like quantity determined by astronomical obser- 

 vation ; but as no such observations had been made in any of 

 the instances, the argument could not be carried so far. 



There is hoAvever good reason to think that the longitudes 

 computed on the assumed spheroid are very nearly equal to 

 the true quantities, at least in all the perpendicular measure- 

 ments made in England. For all these instances lie very near 

 the meridian of Greenwich ; and in that region we know by 

 experiments on which confidence can be placed, that the lon- 

 gitudes determined astronomically agree with the geodetical 

 computation. Thus it follows from what is shown in this 

 Journal for September 1828, pp. 191, 193, that the difference 

 of longitude between Dunkirk and Greenwich, determined ex- 

 perimentally, is almost exactly equal to the same quantity cal- 

 culated on the given spheroid of revolution ; and I shall now 

 add another similar instance in corroboration of my argument. 



In the Phil. Trans. 1824, Dr. Tiarks gives the difference 

 of longitude between Dover and Falmouth, determined by 

 chronometers, equal to 25' 28""42 in time ; and the difference 

 of longitude between Falmouth and the observatory at Ports- 

 mouth, equal to 15' 45""51 from a mean of two results: — we 

 therefore have the difference of longitude between Dover and 



the 



