6S2 On the IWror discovered in the Nautical Almanac. 



By an attention to these deductions, it will be seen that 

 the obliquity of the ecliptic at the summer solstice, for the - 

 year 1S02, comes out ll",l more than at the winter sol- 

 stice; and that the obliquity at the winter solstice differs 

 but little from that given by Dr. Maskelyne in the preface 

 of the Nautical Almanac for the year 18)2, and- used in the 

 calculation of the Almanac for that year. In a former 

 communicalion to this journal, I took notice of the uncer- 

 tainty, not only in the mean obliquity of the ecliptic, but in 

 the quantity of secular variation, with a hope that astrono- 

 mers would shortly be gratified in a knowledge of its actual 

 state, both as to decrease and quantity, from a series of ob- 

 servations to be made with an instrument invented and ex- 

 ecuted by Mr. Troughton, whose skill as an artist is too 

 well known and appreciated to require any commendation. 

 Astronomers are much indebted to Mr. Pond for his excel- 

 lent observations, and his discovery of the existing error in 

 • the mural quadrant at Greenwich, which, at the same time 

 that ihev added to the accuracy of astronomical deductions, 

 gave birth to the necessity of calling forth the inventive 

 genius and superior excellence in the executing hand of 

 Troughton. 



There can now be but little doubt that the mean obliquity 

 of the ecliptic, as given by Dr. Maskelyne in the Nautical 

 Almanac for 1812, and as deduced by him from the winter 

 solstice, is considerably wide of the truth ; and that it is pro- 

 bable the obliquity is not nmch different from what he has 

 assumed in the subsequent Almanacs. It may, therefore, 

 appear necessary in very nice observations to allow a cor- 

 rection for this difference, though this perhaps will seldom 

 occur*: for the only part of the ephemeris affected by it of 

 consequence, will be the sun's declination and right ascen- 

 sion : the latter, however, is too small to be of importance, 

 even in the nicest observations, as in astronomical observa- 

 tions we only want the daily difference of the sun's right 

 ascension, which will not be sensibly affected b) so small a 

 quantity as that under consideration. I shall, however, give 

 the investigation of a formula for making this correction, or 

 any corrcotion of a similar nature. 



For Declination. 



Let L= the sun's longitude; 0= the apparent obliquity 



• If the whole quanllty of 8 ',3 be allowed for as error, it makes an un- 

 certain v of about one seventh of a mile in the latitude deduced from the 

 siia's deciiriation,and this only at llie time of the toLtices. vi^. June 21st and 

 Dec. 2tst, an uncertainly even in the extreme cases by far too small to be 

 noticed in nautical pra,ctic( , it being- a doubt whether the best sextants are 

 not liable to as great or greater uncertainty. 



of 



