SURVEYING INSTRUMENTS. 



97 



level stands in the middle of the opening, the angle being 

 shown on the divided arc. 



Price, $6.00 and $8.00. 



Dipping Needle. The Fig. 109. 



dipping needle (Fig. 109) 

 is usually a flat oblong 

 piece of steel, about 6 in. 

 long, accurately centred, 

 and balanced previous to 

 being magnetized, and 

 having a slender cylin- 

 drical axis, fixed at right 

 angles through its centre, 

 and moving freely on its 

 supports. The mounting 

 consists of a brass plate, 

 supported by three screws. 

 In the centre of this brass 

 plate is another, concen- 

 tric with the former, and 

 movable round a centre- 

 pin, like the movable plate 

 of a theodolite. To this 

 plate are attached two levels, the one placed at right an- 



fles to the other, and used to adjust the plate horizontally, 

 'our pieces of brass support the circular case of the dipping 

 needle, the two faces of which are of plate glass, within 

 which two straight bars of brass are firmly fixed across in n 

 horizontal direction; other two brass pieces are fixed b\ 

 screws to the centre of the bars, and carry two finely polished 

 planes of agate, on which the axis of the needle rests, and 

 upon which it turns with very little friction. There is a 

 contrivance inside the box, connected with a small knob 

 outside, by which the observer can lift, by means of Y's, 

 the needle from the agate plates, or lower it upon them at 

 pleasure ; the Y's being carefully adjusted so as always to 

 leave the axis of the needle on the same part of the agate 

 planes, and in the centre of the graduated circle, and from 

 which graduated circle, the angle the needle makes with 

 the horizon is indicated ; this circle is usually divided to 

 half or quarter degrees, and there is sometimes a vernier 

 attached to the end of the needle, and also a reading micro- 



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