CHAP. VIII. [ 20]
To frame a MERIDIONAL DYAL, either to East or West.
A Scheme of both these are already treated of [ 30] in Chapter III, Problem II. But first of the Meridian East Dayl: First, Draw the Line ab, Horizontal, or Parallel to the Horizon. Secondly, From a, draw a slight Circle at any opening of the Compasses, beginning from the Horizontal Line, namely from h, and that towards the right side where the Oriental Dyal is to be drawn. Thirdly, Seek, or imagine in the Instrument the like Arch, that is, at the same opening of the Compass, then in that Arch cut off the Complement of the Poles [ 40] Elevation, or which is the same, the Elevation of the Aequator; and transfer that Interval to the Circle drawn from the point d, and let the Arch be cut off; For Example, in the point e. Fourthly, through that point e, and through the extremity of the Horizontal Line a, b, draw the Line e, a, which will be in the Head of an Aequinoctial Line, because therein are to be designed the Horary Points. But before the marking of these points, draw another Line Parallel to that Aequinoctial. Fifthly, About the upper part of the Aequinoctial [ 50] Line mark at pleasure the Hour 6, and also the like point in the opposite Parallel, so as that be∣tween these points the Line of the Hour 6, be made a right Angles. (Note that this Hour 6 is the beginning of the Hours in this kind of Dyal, and that their Intervals are taken from it. Also, that the place of the Style is therein, which is to be fixt in the upper part of that Line, that is, where it goes under within the Horizontal.) Sixthly, From the Horizontal Dyal, or from the Instru∣ment [ 60] transfer hither the Intervals of the Hours, and first the Distance of the Hour I, from XII; which is found in the Horizontal, or in the Instrument, will be in this place the Interval of the Hour V, on the one side, and on the other, of the Hour VIII; also the Distance of the Hour XII from II, will mark the Hour VIII; also the Distance of the Hour III, will be the Interval of the Hour IX; and likewise the Distance of the Line of the Hour VI, of this Dyal; and the same proporti∣onal Distance is to be between the Hour V, and XI.
It is to be understood that the same Horary Points which are mark't in the Aequinoctial Line, are also to be mark't in the Parallel, opposite 〈◊〉〈◊〉 it, to the end the Parallel Horary Lines may be commodiously drawn from Point to Point.
Note, Secondly, That the first Hour of the Sun, which appears in this sort of Dyal, is V, and so forwards as far as II, inclusively; for XII cannot well appear, when as the Dyal is supposed to be within the Meridian Circle.
Note, Thirdly, That the Style in this Dyal is a right Index of a determined Altitude, and is fetcht from an Interval which is found between the point of the Hour VI, and the point of the Hour IX.
Now as for the Meridian West Dyal, it differs not much from the Meridian East Dyal, except that the occult Arch, which is for distinguish∣ing the point of the Aequinoctial Line is drawn from the left side; and that in the West Meridi∣dian the Evening Hours are to be drawn begin∣ing from the First to the 7th inclusively, with the like Intervals, as in the East Meridian: but the First is in the lower place, and so the rest in their order. Moreover, the Hour VI, is the place of the Style, in like manner as the East Meri∣dian, and hath the same Altitude.