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The Analogie or Proportion is,
As the Co-sine of B C 78 degr. 30 min. is to the Co-sine of A 66 degr. 30 min.
So is the Radius to the Sine of the Angle at the Perpendicu∣lar B.
HAving drawn your Quadrant A B C, take from your Line of Chords 78 degr. 30 min. the Complement of the given Side B C, and set it from B to a. Also take from your Chords 66 degr. 30 min. the Complement of the given An∣gle at A, and set them from B to b, and through the Points a and b draw the Lines a c and b d parallel to B A. Then ta∣king in your Compasses the distance A c, set one foot in C, and with the other foot describe the Arch p, and draw the Line A O so that it onely touch the Arch p. Likewise, take in your Compasses the distance A d, and with that distance, setting one foot upon the Line A C, move it gently along that Line till the other foot, being turned about, do onely touch the Line A O. So shall you finde the point of the Com∣passes to rest in the Point e, through which Point draw the Line e o parallel to B A. Then measure the distance B o up∣on your Line of Chords, and you shall finde it to contain 69 degr. 22 min. the quantity of the enquired Angle at B.