70 Inflmmental InflruSlions 



true North and South Line 5 to this Line at 

 right Angles, at Point E draw another Line, 

 for the Line of Eaft and Weft. 



v-i ^4: N^'-J ^''<, ^"'4 ■^■'^ v'-i ^"'4 >>"<: v'/xvy-j X '/ .v"Zi<-/XNV oj ^y/-/; vy^/.v/^ v/y vvx N^•/x s^/x syv sy/x 



'/>;^ *.? ^vU^^;.^ ?.;^ vi ^/;.< U ^A^ '.;.^j^^ >^^^^ 



T/?^ 1' H E O D O L I T E. 



AS the Horizon (which is a great Circle of 

 'the Sphere) is, according to the heft of 

 OWervations, divilible into 36c equal Farts 

 caird Degrees, each Degree containing 60 o- 

 tlier Parts calTd Minutes : So the Theodolite 

 is a round Inftrumenr, reprefenting the Hori- 

 zon, a'nd divided into 360 Degrees number'4 

 at every tenth' Degree, and each Degree fub- 

 divided into Halves and Quarters anfwering to 

 goand 1 5 Minutes. It has two Diameters crof- 

 ling each other at right Angles,whofe Extremi- 

 ties reprefent the 4 Cardinals Points N. E, S. 

 W. On the Center of thefe plays a moveable 

 Diameter, whole Ufe is to give any intermedi- 

 ate Number of Degrees, with one of the fix'd 

 Diameters, where the Qiiantity is lefs, or 

 more than 90. 



The LinCjWhich reprefents North and South, 

 paffes thro' the Points of 180 and 360, and 

 that Line (and, confequently, the Inftrument) 

 is rightly plac'd, when, by moving about the 

 Inftrument, the Needle in the Box plac d in 

 the Center, lies on the fame Points. ^ -I^' 



To the fix'd Diameter, cutting into 180 

 and 360 Degrees, and to the moveable one 

 (which for the Future we will call J he Index) 



are 



