Footnote 72
Supp. Prop., §46. This article is probably not Chaucer's.  It is found in MS. Bodley 619, and in MS. Addit. 29250.  The text is from the former of these, collated with the latter.  What it asserts comes to this.  Suppose it be noted, that at a given place, there is a full flood when the moon is in a certain quarter; say, e.g. when the moon is due east.  And suppost that, at the time of observation, the moon's actual longitude is such that it is in the first point of Cancer.  Make the label point due east; then bring the first point of Cancer to the east by turning the Rete a quarter of the way round.  Let the sun at the time be in the first point of Leo, and bring the label over this point by the motion of the label only, keeping the Rete fixed.  The label then points nearly to the 32nd degree near the letter Q, or about S.E. by E.; shewing that the sun is S.E. by E. (and the moon consequently due E.) at about 4 A.M.  In fact, the article merely asserts that the moon's place in the sky is known from the sun's place, if the difference of their longitudes be known.  At the time of conjunction, the moon and sun are together, and the difference of their longitudes is zero, which much simplifies the problem.  If there is a flood tide when the moon is in the E., there is another when it comes to the W., so that there is high water twice a day.  It may be doubted whether this proposition is of much practical utility.