360 On the Figure of the Earth. 



thereby be enabled finally to deduce the true and exact figure of 

 the earth. 



§ 18. Although it appears, from the different measurements of 

 the degrees of the meridian, that the figure of the earth is not 

 regular, still it is possible that the irregularities do not belong 

 so niucli to the figure, or the radii, as to the nature of the upper 

 strata, the different density of which may occasion the concealed 

 error in the perpendicularity of the instruments : an error which 

 (as I have elsewhere shown) may be quite sufficient to reconcile 

 all the disagreements between the measurements hitherto taken*. 

 Consecjueutly the variations iu the parallax in different latitudes 

 might very well proceed with as much regularity as appears to exist 

 in the variation of gravity, and iu the length of the pendulum. 



§ 19. It is in the power of every principal Academy materially 

 to assist in such a discovery, by two methods. First, in regard 

 to times past, to collect together, from all quarters, the observa- 

 tions of occultations stated to have been made in a given interval; 

 for instance, in the last ten years : and to employ some calculator 

 to select and compute all those which are ])roperfor showing the 

 variation of curvature at different places. Secondly, with respect 

 to the Jut i/re, to insert in the Ephemerides accurate notices of 

 those places or districts where it would be most important that 

 any occultation should be observed (particularly of the principal 

 stars), in order that it might serve to apprize, and excite the at- . 

 tention of such astronomers as might be favourably situated them- 

 selves, or contiguous to more advantageous situations. It appears 

 to me that such notices would l)e more important and useful than 

 those of the phases of solar eclipses, about which the calculators 

 of Ephemerides are in the habit of taking so much trouble. 



KND OF THE MEMOIR. 

 [The Appendix will be given in our next.] 

 * [ndependent of the deviations arising from the causes here alluded to, 

 the plumb-line has been sometimes known to be attracted towards the sides 

 of the ffluss vessel, containing the weight, as jiowerfully as gold-leaf tovvards 

 an electrical tube. The remark appears to have been made by M. Flauger- 

 gues. " Astronomers ought to avoid using a. glass vessel for the water in 

 which the weight of the plumb-line is suspended ; for, I have observed twice, 

 in one year, a singular deviation in the plumb-line occasioned by the attrac- 

 tion of the ball of the plumb-line towards the side of the vessel in which it 

 was suspended. This ball was drav,-n towards the side with as much rapi- 

 dity as gold-leaf is attracted by an electrified tube : and I was obliged (in or- 

 der to destroy the eftect of this spontaneous electricity, so as to enable me 

 to take equal altitudes) to put a coating of sealing-wax upon the ball. But, 

 since I have substituted a metal \essd, this singular phainomenon has not 

 again occurred." Connalssance dca Terns, Anncc xiii. page 413. Although 

 such a powerful impulse as this may not often have occurred, yet it is pos- 

 .sible^that slight deviations from the perpendicular may frequently have arisen 

 trom the cause here alluded to ; and which may account for some anomalies 

 which have been remarked in the observations made in the course of the sur- 

 veys. B. 



