PLOTTING QUADRANT, LEVEL, AND CALCULATOR. QQJ 



Question third. — Tojinda mean proportional as between 6C0 



and 200. 

 This depends on the well known property of a right angled 



triangle. 

 Set the perpendicular on the hase line, at the distance of To find a mean 

 lialf of the difference of the two numbers [i. e. eoo — « o.o); this P'-oportional. 

 done, raise the perpendicular, and move either of the lifnbs 

 till the extremity of the 7)<?r/)e«t/ic?//ar intersects thereon at 

 half the sum of the numbers, being 400. This done, the 

 height of the perpendicular will show the proportional re- 

 quired, being- 347» 



Note. — On the plate on which the perpendicular slides 

 will be found Noniuses for subdividing the divisions on the 

 base or perpendicular into 10 divisions. 



Reference to the Engraving of Mr. Saltnon^s Geometrical 

 Quadrant and Staff, Plate V. 



Tig. I represents the face of the quadrant, on which A is Explanation of 

 the fixed base line ; B the moveable perpendicular; C the 

 upper limb ; D the lower limb ; E the arc ; F the nut, which 

 moves the perpendicular by means of a rack and pinion. 



G, a spring to keep the perpendicular steady ; H, a screw 

 for fixing the joint of the staff; a the eye piece, or sight, with 

 a small hole in its centre; I, I, T, the sights for direct vision, 

 consisting of only a small slit in each. When objects are to 

 be viewed by reflection, as with a Hadley's quadrant ; the 

 sight a at the centre is taken off, and the sight b with a mir- 

 ror, shown at Fig. II on rather a larger scale than the for- 

 mer, must be substituted. 



Fig. III. — K is the staff, the mode of applying which to 

 support the instrument when in use is shown by the same 

 letters in the other figui-es ; L is the screw, by which the staff 

 is fixed firmly in the ground. 



Fig. IV represents the back of the quadrant; M M are 

 the screws, by which the upper and lower limbs are fastened 

 after taking an observation. 



O, a spirit level ; H the tightening screw for the joint be- 

 fore noticed ; P the socl<et attached by its joiiit and tighten- 

 Q 2 inP 



