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



with its centre by four radial bars, a a. &c. The centre of 

 the metal is removed, and a circular disk of glass fixed in 

 its place, on which are drawn two lines crossing each other 

 at right angles, and dividing the small circle into four quad- 

 rants, the intersection of the lines denoting the centre of the 

 protractor. When the instrument is used for laying down 

 an angle, the protractor must be so placed on the paper 

 that its centre exactly coincides with, or covers the angular 

 point ; which may easily be done, as the paper can be seen 

 through the glass centre-piece. 



Round the centre, and concentric with the circle, is fitted 

 a collar, b, carrying two arms, c c, one of which has a 

 vernier at its extremity adapted to the divided circle, and 

 the other a milled-head, d, which turns a pinion, working 

 in a toothed rack round the exterior circle of the instrument ; 

 sometimes a third arm is applied at right angles to the 

 other two, to which the pinion is attached, and a vernier 

 can then, if required, be applied to each of the other two, 

 and it also prevents the observer disturbing that part of the 

 instrument with his hand when moving the pinion. The 

 rack and pinion give motion to the arms, which can thus be 

 turned quite round the circle for setting the vernier to any 

 angle that may be required. Upon a joint near the extremi- 

 ty of the two arms (which form a diameter to the circle) 

 turns a branch, e e, which for packing may be folded over 

 the face of the instrument, but when in use must be placed 

 in the position shown in the figure : these branches carry, 

 near each of their extremities, a fine steel pricker, the two 

 points of which, and the centre of the protractor, must, for 

 the instrument to be correct, be in the same straight line. 

 The points are prevented from scratching the paper as the 

 arms are moved round, by steel springs, which lift the 

 branches a small quantity, so that, after setting the centre 

 of the protractor over the angular point, and the vernier in 

 its required position, a slight downward pressure must be 

 given to the branches, and each of the points will make a 

 fine puncture in the paper ; a line drawn through one of 

 these punctures and the angular point will be the line 

 required to form the angle. 



Any inaccuracy in placing the centre of the protractor 

 over the angular point may easily be discovered, for, if in- 

 coriectly done, a straight line " drawn through the two 

 punctures in the paper will not pass through the angular 

 point, which it will do, if all be correct. 



