The Third Book of the Cross-Staff
It was the manner of the Ancients to divide the Day into 12 equal Hours, and the Night into 12 equal Hours, and so the whole Day and Night into 24 Hours. Of these 24, those which belonged unto the day were either longer or shorter (excepting the two Equinoctial Days) than those which belonged to the Night; and the Summer Hours always longer than the Hours in the Winter, according to the lengthening of the Days, whereupon they are called the Old Unequal (and by some the Planetary) Hours.
To express these in the former Planes, first draw the common Hour-lines, the Equator, and the Tropicks, as before: Then describe two occult Parallels of the length of the Day, one for 9 Hours, the other for 15 Hours; for so you may draw a straight Line for the first unequal Hour through 5 ho. 45 m. in the parallel of 15, and through 8 ho. 15 m. in the Parallel of 9. This straight Line shall pass directly through 7 ho. 0 m. in the Equator, and so cut off a twelfth part of the Arks above the Horizon, both from these two Parallels and the Equator; and being continued unto the Tropicks, it shall also cut off about a twelfth part from them, and all the rest of the Parallels of Declination, without any sensible error.
In like manner you may draw the second unequal Hour through 7 hour in the Parallel of 15, through 8 hour in the Equator, and through 9 hour in the Parallel of 9, and so the rest as in the Table.
And of these unequal Hours you have further Example in the Diagram belonging to the Polar Declining Plane.
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