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359—Free Tangent Screw.—There is always a risk of a tangent screw of any fixed kind producing a certain amount of strain upon the instrument, therefore, where practicable, it should be made free. The illustration, Fig. 150, shows the form of free tangent the author now applies to many instruments. The centre stud is clamped to the lower part of the instrument by the screw shown in dotted lines. To the left hand a piston containing a spiral spring carries a pressing-rod against which the screw to the right hand works.
Fig. 150.—Free tangent adjustment.
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360.—Loss of Time by Wear of the nut is variously taken up when no spring is used. One plan was shown of splitting it up. A plan common in Germany is to make the nut in two pieces, which are brought up by two screws. This is a very effective plan. The author has found a tumbling piece arrangement also effective. Fig. 151, S section of tangent screw, T tumbling piece moved by the adjusting screw, shown above, for wear of the tangent screw. This adjusting screw A should be tapped tight without oil, and put together dry to prevent its receding by pressure.
Fig. 151.—Tumbling piece adjustment for wear of tangent screw.
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361.—Hypotenuse and Base.—Other trigonometrical values besides the division of the circle into equal parts are occasionally placed on instruments for special purposes. The most common of these is the scale of difference of hypotenuse and base, which is generally placed upon the back of the vertical arc of a theodolite and upon some dials and clinometers. The division for this purpose is generally done by hand. The scale gives a percentage difference for certain angles. Thus when used with chain measurement, it gives the number of links of the chain to be deducted per chain of 100 links for the inclination of land that the theodolite or other instrument indicates in following the surface contour.