G4 BEXJ. PIKE'S, JR.. DESCRIPTIVE CATALOGUE. 



fields, as well as in surveying them. It is used by setting it 

 on the top of a post, fence, &c. Its diameter is 4 inches. 

 Packed in a mahogany case, Price $5.00. 



Fig. 91 



The Graphometer, and Four Sighted Tfieodolite. (Fig. 94, 

 as above.) The error arising from the use of an instrument, 

 where the whole dependence is placed on the needle, being 

 frequently influenced by local attractions, has rendered it 

 necessary for some other method to be employed to measure 

 angles with accuracy ; among these the common theodolite 

 with four sights has taken the lead. 



It is simple in its construction, and easy in its use. 



The annexed figure represents the graphometer, a brass 

 plate or part of a circle about twelve inches in diameter, 

 graduated on its edge from to 180 degrees; in the open- 

 ing between the moving centre and the graduated arc, is a 

 compass about four inches in diameter ; two sights are fixed 

 on the graduated arc, one at a~nd the other at 180. Per- 

 pendicular to the plane of the instrument, there is a movable 

 limb attached to the limb of the arc, but a little shorter, and 

 having the extremities slanted off, one of which forms a 

 nonius, subdividing the degrees on the limb to minutes, and 

 having two sights, one at each end ; in each sight there is a 

 large and a small aperture, placed alternately, the large 

 aperture in one sight being always opposed to the narrow 

 aperture in the other ; underneath the plate is a spring to 

 fit on the pin of a ball and socket, which fixes it the single 

 or three legged staff, as may be required. In the figure the 

 ball and socket are represented detached from the instrument. 



The Four Sighted Theodolite is the same instrument, ex- 



