3 io Of the Drill-Boxes. Chap. XX. 



When Two oppofite Sides, or Surfaces, are in- 

 clined to each other upwards, I call that Inclination 

 a- Bevel ; but when they are inclined downwards, I 

 cali it a Bevel revers- d. 



The Line a e 7 being the Bottom, or Bafe, of the 

 Hinder-end D, by being longer than the line / w, 

 ihews that the Mortife is bevel. 



The Two prick'd Lines m n and / o 9 with the. 

 Line lm y and Fart of the Line a e, make a rectangled 

 Parallelogram, which fhews the exact Depth of the 

 Mortife, and forms on each Side of it a reftangled 

 Triangle, the one m e #, and the other / o a •, which 

 Triangles being fimilar and equal, and their acutq 

 Angles at / and m being each of Four Degrees, make 

 the whole Bevel, or Inclination of the Sides of the 

 Mortife, to be of Eight Degrees, their Hypothenufes 

 being the fame with the Sides of the Mortife. 



This End D, being raifed up to its Place, will be 

 at right Angles with the Plane of the Top and of the 

 Bottom of the Mortife ; which, being both rectan- 

 gled Parallelograms, prove that Bevel, or Angle of 

 Inclination, to be the fame from one End to the 

 other of the Sides, which Sides are the Hypothenufes 

 of thofe Two Triangles : But this could not be proved 

 by the Triangles in the oppofite End C j becaufe the 

 Bafes being the fame with the other, and having their 

 Legs longer, the vertical Angles at k and/ are more 

 acute. The Legs are longer; becaufe the End C, 

 when in its Place, is not at right Angles with the Top 

 and Bottom of the Mortife, as the End D is. 



The next thing to be defcribed in the Mortife, is 

 the Bore, great Hole, or Perforation ; which is beft 

 ihewn in the Side of a Mortife of a Wheat-drill, 

 "being larger, as in Fig. 3. wherein c e b d is the! 

 great Hole, and is a Section qf an hollow Cylinder, 

 that paries through the Mortife, with its Axis parallel 

 to the Edges of the Ends of the Mortife: This 



Cylinder, 



