SURVEYING INSTRUMENTS. 65 



cept that the circle is entire, and the compass is placed in 

 the centre of the circle. 



Cautions in using the instrument. 1. Spread the legs that 

 support the theodolite rather wide, and thrust them firmly 

 into the ground, that they may neither yield nor give un- 

 equally during the observation. 2. Set the instrument 

 horizontal. 3. Screw the ball firmly in its socket, that in 

 turning the index the theodolite may not vary from the ob- 

 jects to which it is directed. 4. Where accuracy is required, 

 the angles should always be taken twice over, oftener where 

 great accuracy is material, and the mean of the observation, 

 must be taken for the true angle. 



To measure an angle with the Theodolite. Let ABC repre- 

 sent the angle to be measured ; place the theodolite over 

 the angular point A, and direct the fixed sights along one of 

 the lines, till you see through the sights the point B ; at this 

 screw the instrument fast ; then turn the movable index till 

 through its sights you see the other point, c ; then the de- 

 grees cut by the index upon the graduated limb or ring of 

 the instrument show the quantity of the angle. 



The fixed sights are always to be directed to the last 

 station, and those of the index to the next. 



Price, 8 inch, in case, $15.00. 

 " 12 " " $30.00. 



The Quadrant. (Fig. 95, next page.) The Quadrant 

 consists of an arc firmly attached to two radii, or bars, 

 which are strengthened and bound together by two braces. 



Of the Index. The Index is a flat bar of brass attached 

 to the centre of motion. At the lower end of the index 

 there is an oblong opening ; to one side of this opening a 

 nonius scale is fixed to subdivide the divisions of the arc. 

 At the bottom, or end of the index, there is a piece of 

 brass, which bends under the arc, carrying a spring to make 

 the nonius scale lie close to the divisions ; it is also fur- 

 nished with a screw to fix the index in any desired position. 

 The best instruments have an adjusting screw fitted to the 

 index, that it may be moved more slowly, and with greater 

 regularity and accuracy than by the hand. The circular 

 arcs on the arc of the quadrant are drawn from the centre 

 on which the index turns. The position of the index on ths 

 arc, after an observation, points out the number of degrees 

 and minutes contained in the observed angle. 

 6* 



