98 PHILOSOPHICAL TRANSACTIONS. [aNNO l666i 



these times, yet there is a compound of two that may do it : which is, the 

 inequality of the natural day (I mean that of 24 hours from noon to noon) 

 arising at least from a double cause ; either of which singly would cast it upon 

 other times, but both jointly on those. 



It is commonly thought, how unequal soever the length be of the artificial 

 days, as contradistinguished to nights, yet that the natural days, reckoning 

 from noon to noon, are all equal : but astronomers know well that even these 

 days are unequal. 



For this natural day is measured not only by one entire conversion of the 

 equinoctial, or 24 equinoctial hours, which is indeed taken to be performed 

 in equal times, but increases by so much as answers to that part of the sun's (or 

 earth's,) annual motion as is performed in that time. For when that part of 

 the equinoctial which with the sun was at the meridian yesterday at noon, is 

 come thither again to-day, it is not yet noon, because the sun is not now at the 

 place where yesterday he was, but is gone forward about one degree more or 

 less, but we must stay till that place where the sun now is comes to the meri- 

 dian before it be now noon. 



Now this additament, above the 24 equinoctial hours, or entire conversion, 

 of the equinoctial, is upon a double account unequal : First, Because the sun, 

 by reason of its apogee and perigee at all times of the year, dispatches in one day 

 an equal arch of the ecliptic, but greater arches near the perigee, which is about 

 the middle of December, and lesser near the apogee, which is about the middle 

 of June; as will appear sufficiently by the tables of the sun's annual motion. 

 Secondly, Though the sun should in the ecliptic move always at the same rate, 

 yet equal arches of the ecliptic do not in all parts of the zodiac answer to equal 

 arches of the equinoctial, by which we are to estimate time; because some 

 parts of it, as about the two solstitial points, lie nearer to a parallel position to 

 the equinoctial than others, as those about the two equinoctial points, where 

 the ecliptic and equinoctial do intersect ; whereupon an arch of the ecliptic, near 

 the solstitial points, answers to a greater arch of the equinoctial, than an arch 

 equal thereunto near the equinoctial points ; as doth sufficiently appear by the 

 tables of the sun's right ascension. 



According to the first of these causes, we should have the longest natural 

 days in December, and the shortest in June, which if it did operate alone, would 

 give us at those times two annual high waters. 



According to the second cause, if operating singly, we should have the 

 longest days at the two solstices in June and December, and the two shortest at 

 the equinoxes in March and September; which would at those times give oc- 

 casion of four annual high waters. 



