of the Earth ly means of Occultations of the fixed Stars. 359 



the errors of the elements made use of in the calculation be con- 

 founded with that difference. But these are precisely the occa- 

 sions most favourable for the observations with which these par- 

 ticular occultations may be compared ; and by which (as I have 

 said) the errors of the tables are corrected, and the geographical 

 longitude determined. 



§ 16. It is true that the conditions here required, for ascer- 

 taining the flattening of the poles, cannot be verv frequently ob- 

 tained : but, if we look out for them with the diligence that the 

 importance of the object demands, we shall perhaps meet with 

 them more frequently than we imagine. For, I mav venture to 

 assert that there does not pass a month without the occurrence 

 of an occultation of some star whose position is well known: and 

 there is no occultation that will not afford, to some part or other 

 of the earth's surface, the conditions required *. If only once 

 out of twenty times they should occur, in a place where there is 

 an astronomical observatory, the question as to the compression, 

 and also as to its quantity, would be very soon determined. 



§ 17. For the solution of such questions, for which immense 

 sacrifices have hitherto been made (for instance, in the measure- 

 ment of the degrees of the meridian), it surely is not requiring too 

 much that the trifling expense should be incurred of enabling as- 

 tronomers to travel to places more favourably situated for making 

 observations of such occultations. This would be an undertaking 

 worthy of a sovereign who wishes to distinguish himself as a pa- 

 tron of science. We should, by such means, gradually arrive at 

 a knowledge of the relative length of the terrestrial radii, in a 

 great number of places : and, it is most probable that we might 



In the Appendix I have given other tables, showing the differences that 

 would arise from varying the latitude of the place, the height of the moon, 

 and the quantity of the earth's compression : whereby the reader may be bet- 

 ter able to judge of the maximum of difference which would arise under the 

 most favourable circumstances. B. 



* Since tliis was written, the positions of most of the zodiacal stars have 

 been determined with a degree of accuracy sufficient for the purposes de- 

 tailed in the memoir. The labours of CcignoU himself, of P'laxzi, Harding, 

 Zach, and Bessell have contributed much to this end : so that wc may now 

 safely assert that scarcely a night passes " without the occurrence of an 

 occultation of some star whose position is well known." Nearly forty years 

 ;igo Messier made the following remark, at the close of a numerous list of 

 observed occultations : " We see by this collection of occultations how many 

 new ones I have observed, in the first quarter of the moon, on the dark 

 limb ; which are distinguished with the greatest precision. They are fre- 

 quent, and much prefcnible to the observations of Jupiter's satellites, or 

 lunar eclipses, for determining the longitude. It were much to be wished 

 that the conductors of our Kphemcridcs should announce, for the first part 

 of each lunation, the immersions of stars of even tlie Jtli, i^th and 9th ma.g- 

 iiitudc, which arc as readily o!)Scrved as those of the 1st, 2nil and '.in\ mag- 

 nitude." Connaissancc dvs Terns, yinnvc \'m. page iH!). B. 



Z 4 thereby 



