224

Mechanics.

pi. IX.Fig. 6.

Fig. 7.

because they all require the same Strength of Wind,which cannot otherwise be had.

15. That these Things may be better understood, Ishall premise the following Definition of some Geome-trical Lines and Figures, which are absolutely necessaryto a compleat Knowledge of the modern MechanicalPhilosophy.

16. I take it for granted, that the Reader knows,that if on any Point C, taken in the Right Line A B,a Circle ADEF be described, the Point C is call’dthe Centre, and AE the Diameter of the Circle : Towhich I shall add, that AC, orCE, is call’d the Ra-dius of the Circle, which is the same Thing as the Se-midiameter.

17. If the Circle be divided into four equal Parts,AD—DE—EF—FA, by the two Diameters AE, DF;then each of the Areas ACD, DCE, ECF, FCA,are call’d Quadrants, or Quarters of the circular Space;and the Parts of the Circle AD, DE, E F, F A,are call’d §)uadrantal Arches, or Quarters of the Circle.

18. Every Circle, great or small, is supposed to be di- ;

vided into 360 equal Parts, call’d Degrees-, consequent- j

ly each Quarter, AD, DE, lAc. will contain 90 of j

those Degrees, as is evidently represented by the large !

Half-Circle of Fig. 7. i

19. If two Lines BC and FC meet in a Point C, j

the Space FCB included between them is call’d anAngle-, and the Measure of that Angle is the Number j

of Degrees contain’d in an Arch El of a Circle de- !

scribed on the Angular Point C, and included between <

the said two Lines BC and FC. Thus the Angle inthe Figure contains 40 Degrees.

20. If a Line, as G C, be drawn through the 90thDegree on the Point C, it will make the Angle on oneSide GCB equal to the Angle GCA on the otherSide, because each is equal to 90 Degrees. Such anAngle is called a Right Angle ; and the Line G C isthen said to be perpendicular to the Line A B.

21. The Angle FCB, which is less than a RightAngle or 90 Degrees, is call’d an Acute Angle ; andthe Angle HCB, which is greater than a Right Angleor 90 Degrees, is call’d an Obtuse Angle. Again; the

Arch