BALANCE LEVEL. 345 



the outer tube. Look through the sights, ami observe the 

 object staiF which the person holds, let him move the slide 

 on the stalF, until you see the hair cut the middle of the 

 slide, on which there is a black line ; then turn the level 

 round, look through the sights, and see if the hair cuts 

 the middle of the slide as before ; if it does, it -vill be level ; 

 but if there be a difference in both ends, the person who 

 holds the staff must set the slide to half that difference. 

 You are then to adjust the level by turning with a key the 

 screw, which moves the balance contained in the bottom of 

 the inner tube. 



Certiacates from Mr. J. W. Gooch, Mr. Charles Lay- ^« "s^d with 

 ton, and Mr. Benj. Holmes, testify, that they have seen in accuracy, 

 use the level invented by Mr. Richard Drew, and that the 

 business is done by it with accuracy and dispatch. 



Reference to the Engraving of Mr. Richard Drezs's Balance 

 Level. Plate X, Fig. 1, 2, 3, 4. 



Fig. 1. The balance level, mounted on a ball and socket The instrument 

 joint, with a tube, a, to fix on a stand. 



Fig. 2. A section, bbccivfo tubes of tin, which slide 

 on a short tube, </rf, placed in the middle, and having an 

 iron wire soldered round it to stiffen it, and to serve as a 

 shoulder. 



e e Two eye pieces, with glass in both, one at each end, 

 and sliding into the tubes b and c, 



ff The balance level, hanging by a sort of staple g, on 

 a point fixed upright on the middle of the bar h (shown in 

 Fig. 3), which is fastened across the tube d. 



ii Two eye pieces sliding into the ends of the level//, 

 and having a narrow^ slit horizontally across the middle, 

 with a hair before each, shown by the dots hh. 



k An adjusting screw, which acts by drawing the piece 

 m^ (which moves in a dove-tail slide), in one end of the 

 tube. 



n The key-hole through which the screw is turned. 

 Fig. 4. An end view of the cue and level, showing the 

 eye pieces i and e, one within the other. 



Vn. Account 



