CHAP. XXI. To finde the perpendicular of any Triangle, the three sides being given.

FIrst, for this purpose lay the threed upon the side of the Quadrat, and rectifie the bead to the first side, then take the second side with your Compasses, and setting one foot in the end of the third side, open the three untill the moving foot of your Compasses, and the bead rest directly both at a point; so shall that parallel, which pas∣seth by the point where the Compasse foot, and the bead meeteth, (parallel to the side wherein the other foot of the Compasses standeth) be the perpendicular required. As for Example; let it be required to finde the perpen∣dicular of the Triangle in the last Chapter. First, lay the threed to the side of the quadrat, and rectifie the bead to the greater side AB, which is 60; then take 45 betwixt Page  37 your Compasses, which is the lesser side BC, and setting one foot in the side of the Quadrat in 75, which is the base AC, with the other open the threed, untill the point of the Compasses and the bead meet both in one point; so shall the parallel distance, betwixt the bead and that side of the Quadrat wherein the foot of the Com∣passes standeth, be 36, the perpendicular required.