( 70 ) 
on the Level; neither is there need of any Computa- 
tion, but only fimply laying the Gun to pafs, in the 
middle Line between the Zenith and the Objcd, and 
giving it its due Charge. Nor is there any great need 
of Inftrumenrs for this purpofe : For if the Muzle of 
the Mortar be turned truly Square to the Bore of the 
Piece, as it uliiaUy is or ought to be, a piece of Look- 
ing glafs Piate applyed parallel to the Muzle, will by its 
Pceflecftion give the true Pofition of the Piece ; the Bom- 
bardeer having no more to do, but to look Perpendi- 
cularly down on the Looking-glafs , alongft a fmall 
Thread with a Plumbet, and to raife or deprefs the Ele- 
vation of the Piece, till the Objeft appear refleded on 
the fame Point of iht Speculum, on v^hich the Plumbet 
falls ; for the Angle of Incidence and Refleftion being 
equal, in this cafe a Line at Right Angles to the Specu- 
lum^ as is the Axis of the Chafe of the Piece, will bifed 
the Angle between the Perpendicular and the Objed: , 
according as our Propofuion requires. So that ic only 
remains by good and valid Experiments to be aiTured 
of the force of Gunpowder, how to make and conferve 
it equal, and co know the effeft thereof in each Piece; 
that is, how far differing Charges will cafl: the fame 
Shot out of it which may moft conveniently be en- 
graven on the outfide thereof, as a (landing Diredion to 
all Gunners, who (hall from thence forward have occa- 
fion to ufe that Piece : And were this matter well af- 
certained, it might be worth the while to make all Mor- 
tars of the like Diameter, as near as may be alike in 
length of Chafe, Weight, Chamber, and all other Cir- 
cumftances. 
This Difcovery that the utmoft Range on an inclined 
Plane is, when the Axis of the Piece makes equal Angles 
with tne Perpendicular and the Obje-ft, compared with 
what I have dem^nftrated of the fame Problem in the 
aforefaid Namb, 179. does lead to and difcover two very 
ready 
