3 2 8 Mifcellanea Curiofa. 



did conftantly bife& the Angle between the Per* 

 pendicular and the Object, as is demonftrated 

 from the Difference and Sum of the Tangent and 

 Secant of any Arch being always equal to the 

 Tangent and Cotangent oP the half Complement 

 thereof to a Quadrant. Having difcovered this, 

 I think nothing can be more compendious, or bids 

 fairer to compleat the Art of Gunnery, it being 

 as eafie to (hoot with a Mortar at any Object on 

 demand, as if it were on the Level ; neither is 

 there need of any Computation, but only fimply 

 laying the Gun to pafe, in the middle Line be* 

 tween the Zenith and the Object, and giving it 

 its due Charge. Nor is there any great need of 

 Inflruments for this purpofe : For if the Muz- 

 zle of the Mortar be turned truly Square to the 

 Bore of the Piece, as it ufually is or ought to be, 

 a piece of Looking-glafs Plate applied parallel to 

 the Muzzle, will by its Reflection give the true 

 Pofition of the Piece, the Bombardeer having no 

 more to do, but to look perpendicularly down 

 on the Looking-glafs, along a fmall Thread 

 with a Plumbet, and to rahe or deprefs the Ele- 

 vation of the Piece, till the Object appear re- 

 flected on the fame Point of the Speculum* on 

 which the Plumbet falls ; for the Angle of Inci- 

 dence and Reflection being equal, in this Cafe a 

 Line at Right Angles to the Speculum* as is the 

 Axis of the Chafe of the Piece, will bifedt the 

 Angle between the Perpendicular and the Ob- 

 ject, according as our Proportion requires. So 

 that it only remains by good and valid Experi- 

 ments to be aflured of the Force of Gunpowder, 

 jaow to make and confervc it equal, and to know 

 the Effect thereof in each Piece; that is, how 

 jar differing Charges will caft the feme Shot out 



