The description and use of the universall quadrat.: By which is performed, with great expedition, the whole doctrine of triangles, both plain and sphericall, two severall wayes with ease and exactness. Also the resolution of such propositions as are most usefull in astronomie, navigation, and dialling. By which is also performed the proportioning of lines and superficies: the measuring of all manner of land, board, glasse; timber, stone. &c.

Stirrup, Thomas.

Table of contents | Add to bookbag | How to cite

The First Book.

<< Previous section Next section >>

CHAP. XIX. To finde the perpendicular of an equilater Tri∣angle, the sides thereof being given.

AN equilater Triangle is that, which hath all his three sides equall; for I neede not stand to define what a Triangle is, seeing it is so well knowne to be a figure made of three lines, either Right or Sphericall, but Page 33 here I speak of Right lined Triangles. Now to finde the length of the perpendicular, which is a line falling from the opposite angle to the base, making Right Angles therewith; lay the threed upon the side of the Quadrat, and rectifie the bead to the length of one of the sides; then opening the threed, untill the bead fall upon the right parallel of halfe the base; so shall it shew the con∣trary parallel of the perpendicular.

[illustration] [diagram of the measurement of an equilateral triangle (ABC)]

Let the Triangle ABC, be given, wherein it is re∣quired to since the length of the perpendicular AD: First, lay the threed upon the side of the Quadrat, and rectifie the bead to 8 the length of the side AB, or AC; then open the threed, untill the bead falleth upon the Right parallel of 4, the one halfe of the base BC, which is BD, or CD; so shall the bead cut the contrary pa∣rallel of 6 9/10 which is the length of the perpendicular AD, which was required.

Thus may you likewise finde the perpendicular of an Isoscheles Triangle, which is a Triangle of two equall sides; for, first placing the bead to one of the sides, then Page 34 opening the threed untill the bead fall on the Right paral∣lel of one halfe of the base; so shall it shew the contrary parallel of the perpendicular.

<< Previous section Next section >>