62 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



their position, perpendicular to the plate, by milled-head 

 screws, and may be detached for convenient transportation ; 

 in each sight there is a large and small aperture, or slit, the 

 one over the other ; these are alternate, that is, the large 

 aperture being above the smaller in one of the sights, and 

 below it in the other ; a fine piece of sewing silk is fastened 

 vertically through the middle line of the large slit, through 

 small holes for the purpose. Under the compass box is a 

 socket to fit in the pin of the staff ; the instrument may be 

 turned around on this pin, or fixed in any situation by the 

 milled-head screw ; it may also be readily fixed in a hori- 

 zontal direction by the ball and socket of the staff, moving 

 for this purpose the box, till the ends of the needle are 

 equidistant from the bottom, and traverse or play with 

 freedom. 



There are usually one or two levels on the plate of the 

 instrument, for more accurately finding a horizontal position. 

 A spring is also placed within the box, having a milled-head 

 screw acting against it, by which the magnetic needle can 

 be lifted off the centre pin and the cap pressed against the 

 glass, to preserve the point of the centre pin from being blunt- 

 ed by the continual friction of the cap of the needle. The 

 most improved instruments have a loose plate, to which is at- 

 tached the compass box, having a vernier over a graduated arc 

 on the face of the brass plate ; by this contrivance the compass 

 box may be moved about its centre without moving the 

 plate to which the sights are attached ; a long screw with a 

 milled head being on the opposite side of the compass box, for 

 the purpose of giving a slow motion when required. Sup- 

 pose the needle to rest between two of the divisions on the 

 graduated circle ; by this vernier the number of minutes 

 contained between the needle and either of the divisions is 

 indicated. The sights also are improved, having small slits 

 in both the upper and lower part, in which holes are placed 

 alternately, for taking sight through. 



To use the Circumferentor, or Surveyor's Compass, Sup- 

 pose a given angle to be measured ; the instrument being 

 fixed on the staff, place its centre over one of the points of 

 the angle, set it horizontal by moving the ball in its socket 

 till the needle is parallel to the bottom of the compass box, 

 or the levels indicate a horizontal position ; turn the end of 

 khe compass box on which the N or fleur de lis is engraved 

 next the eye, look through the sights to one of the objects 



