Improved Geometrical Plotting Quadrant, 6#c. l"i 



tremity intersect the limb before set, on which, at such in- 

 tersection, will be denoted the radius of the inscribing circle, 

 and at the same time may be seen on the base the radius of 

 the inscribed circle. 



Third Question. — To find a mean Proportional letiveen 600 



and 200. 

 This depends on ths well-known Property of a right-angled 

 Triangle. 

 Set the perpendicular on the base line at the distance of 

 half of the difference of the two numbers {/. e, b0 ° ~ eoci ^ . 



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

 limbs till the extremity of the perpendicular intersect thereon 

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

 height of the perpendicular will show the propoitional re- 

 quired, being 347. 



Note. On the plate in which the perpendicular slides, will 

 be found Nonius's for subdividing the divisions on the base 

 or perpendicular, into ten divisions. 



Reference to the Engraving of Mr. Salmon's Geometrical 

 Quadrant and Staff, (Plate IV.) 



Fig. 1. represents the face of the quadrant, on which A is 

 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; 1,1, 1, the sights for direct 

 vision, consisting of only a small slit in each. When ob- 

 jects are to be viewed by reflection, as with a Hadlcy's qua- 

 drant, the sight a at the centre is taken off, and the sight b 

 with a mirror, shown at fig. 2. on rather a larger scale than 

 Che former, must be substituted. 



Fig. 3. K is the staff, the mode of applying which to sup- 

 port the instrument when in use, is shown bv the same let- 

 ters in the other figures; L is the screw by which the staff 

 i,i fixed firm in the ground. 



Fig. 4, 



