378 On the Error discovered in the Nautical Almanac. 



on at the Royal Observatory, rendering the accuracy of that 

 datum a circumstance of the greatest importance, was a means 

 which constantly drew ilie doctor's attention to that sub- 

 ject. He had been accustomed to settle the mean obliquity, 

 as well as its secular variation, from observations made on 

 the .sun's zenith distances taken lor ten davs before and af- 

 ter the summer solstice, and reduced to the time of that 

 solstice, instead of takuig a mean of the reductions derived 

 from the two solstices. This he did in consequence of the 

 discrepancy in the results dciived from each solstice ; and 

 as he was unable to say what produced this diflerence, and 

 knowing (at that time) of no causae but the uncertainty of 

 jrefraction, he was induced to give a preference to the de- 

 ductions derived from the sun's zeniih distance at the sum- 

 mer solstice. Here, however, another difHculty presented 

 iticlf to hin> : — By a comparison of the deductions thus ob- 

 tamcd with the same derived from observations made during 

 the hfe of Dr. Bradley, the secular variation of the obli- 

 quitv of the ecliptic came gut much less than the secular 

 variation derived from the late observations, or those in the 

 time of Dr. Bradley with more ancient ones, in which a 

 much longer period had been embraced ; so that the variation 

 of the obliquity of the ecliptic must from such determina- 

 tion appear to be decreasing less than formerly, for which 

 no cause could be assigned. This led Dr. Maskelyne to 

 consider whether he was right in giving more weight to 

 the deductions made from the observations at the summer 

 solstice than those at the winter solstice; and findmg the 

 deductions from the latter agreeing better when compared 

 with these made in the time of Dr. Bradley,'' in giving the 

 secular variation ijearer to what arises from eitlier of those 

 observations when compared with ancient observations, in- 

 duced him at last to adopt them in preference to the deduc- 

 tions deriveri from observations about liie summer solstice. 



It will be recollected that I have already said that Dr. 

 Maskelyne knew of no cause to give preference to either; 

 and it was a circumstance of anxiety lo him to see so great 

 a difference between the results at the two solstices as the 

 Greenwich quadrant gave, without bciivg able to assign a 

 reason. Knowing the care taken in making the Grecnv. ich 

 observations, and their superior accuracy over those made 

 on tlfe con'.ineni. prevented him from rclmquishing hi> de- 

 ductions for an adoption of others which he could not 

 bring his mind to heheve were so much to be depended on. 

 The^susp.nsc which this stale of things induced, was 

 not however to be of Ions; contiiruance. A series of obser- 

 vations made by Mr. Pond, our present aitro;\omer royal, 



on 



