VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 67 1 



their eclipses of the moon, whose circumstances are best related with the tables, 

 he found the computed places of the moon were mostly 5' too forward, and 

 in some considerably more, which he could ascribe to the errors of observation ; 

 but concluded, that the moon's mean motion since that time, must have been 

 something swifter than the tables represent it ; though the disagreement of the 

 observations between themselves is too great to infer any thing from them with 

 certainty in so nice an affair. 



He then compared the 4 well known eclipses observed by Albategnius with 

 the tables, and found the computed places of the moon in 3 of them consider- 

 ably too forward : this, if he could have depended on the longitude of Aracta, 

 would very much have confirmed the opinion that the moon's mean motion 

 must have been swifter in some of the last centuries than the tables make it ; 

 though the differences between these observations and the tables, are not uniform 

 enough to be taken for a certain proof of it. 



Mr. D. could meet with no observations of eclipses to be at all depended on, 

 between those of Regiomontanus and Albategnius, except 2 of the sun and 1 

 of the moon, made at Cairo in Egypt, related in the prolegomena to Tycho 

 Brahe's Historia Ccelestis, p. 34 ; nor any between those of Albategnius and 

 Ptolemy, besides the eclipse of the sun observed by Theon at Alexandria. 

 These eclipses of the sun are the more valuable, because they were observed in 

 places the longitudes and latitudes of which are determined by Mons. Chazelles 

 of the Royal Academy of Sciences, who was sent by the French king in the 

 year 1693, with proper instruments for that purpose. Du Hamel Hist. Acad, 

 p. 309, 395. 



The solar eclipse observed by Theon, was in the 112th year of Nabonassar 

 the day of Thoth, according to the Egyptians, but the 22d day of Pauni, ac- 

 cording to the Alexandrians : he carefully observed the beginning of 2 temporal 

 hours and 50"" afternoon, and the end at 4-j- hours nearly afternoon at Alexandria. 

 Theonis Comment, in Ptol. mag. Construct, p. 332. This eclipse was June 16, 

 in the year of Christ 364. And the temporal hour at Alexandria being at that 

 time to the equinoctial hour as 7 to 6, makes the beginning at 3 equinoctial 

 hours and 18"' afternoon, and the end at 5 equinoctial hours 15"" nearly. 



The eclipses observed at Grand Cairo were as follow. 



The solar eclipse Decemb. 13, in the year of Christ 977, the beginning at 

 8'' 25™, and the end at 10^ 45™, apparent time in the morning. — 2. The solar 

 eclipse June 8, 978 ; the beginning at 2^ 31"", and the end at 4^ 50™, apparent 

 iime afternoon. — 3. The lunar eclipse May 14, 979; but as the middle cannot 

 be known from what was observed of it, he made no use of it in this inquiry. 



That the before-mentioned solar eclipses might be applied to the examination 

 of the lunar motions, he contrived the following method ; which he thinks 



