380 On the Error discovered in the Nautical Almanac. 



from the mean cbliquity given at the beginning of the pre- 

 ceding year : but that it could not arise from that conjecture 

 might have been easily concluded, without other evidence, 

 from knowing that the mean obliquity of the ecliptic re- 

 quires a correction to reduce it to the apparent for every dav 

 in the year. Had this mistake, therefore, originated in one 

 instance, it could not have taken place in all the calculations 

 throughout that year, and by each computer, as each com- 

 puter would have to make all the reductions for himself; 

 but, perhaps, the writer of the article above alluded to may 

 not be aware of the necessity of this reduction, and has 

 adopted the conclusion, that the apparent obliquity of the 

 ecliptic is the same during the whole year. 



Dr. Maskelyne having altered his quantitv of the mean 

 obliquity of the ecliptic at the beginning of the year 1813, 

 ^vas obliged to return bark to hir deductions, derived from 

 the observed zenith distances of the sun at the sun)mer 

 solstice, and again to his deduced secular variation : hence 

 he was compeljed to notice this variation, which he does 

 in his preface to the Nautical Almanac for the year 1813, 

 where he says, " By the summer solsticial zenith distances 

 of the sun of late years (only adding one second to the zenith 

 distances for the error of the mural quadrant, altering its 

 ligure according to Mr. Pond's comparison of the declina- 

 tions of the stars observed with circular instruments, by 

 liimself and other astronomers, by those given by myself at 

 the end of the Greenwich Observations of 1802, and pub- 

 lished in the Philosophical Transactions of 1806), and tak- 

 ing the mean annual diminution of the obliquity of the 

 ecliptic at present to be at the rate of 42'',6 in 100 years, 

 I have assumed the mean obliquity at the beginning of this 

 year to be 23° 27' 51", 3 : these numbers were used in the 

 computation of this ephemeris." 



Dr. Maskelyne here plainly tells us, that he takes the 

 obliquity of the ecliptic from the Greenwich observations ; 

 and althougli he gives the secular variation only 42",6, he 

 assigns no reason for his having taken it so much less than 

 generally stated, — the French making it in their new Solar 

 Tables, 52", 1. 



It is but right, however, that I give in this account such 

 documents as shall confirm the accuracy of the statements 

 above adduced ; fir which purpose I shall state at full lengili 

 the deduction of the observations of the sun's zenith di- 

 stances taken at the Royal Observatory, and the obliquity 

 of the ecliptic thence arising for the year 1802, which ob- 

 servations 



