460 



Popular Science Monthly 



How to Plot Angles Accurately in 

 Fractional Parts of a Degree 



THE following is a simple and 

 accurate method of plotting angles, 

 especially where they are to be inter- 

 preted in fractional parts of degrees 



A method of plotting angles where they are 

 interpreted in fractional parts of a degree 



which would be difficult to read from or- 

 dinary protractors. 



As an example let it be required to lay 

 off an angle of 29°— 21'— 28". By 

 consulting a table of natural tangents we 

 find the tangent of the given angle to be 

 .5625, and as this represents the altitude 

 of a right angle triangle whose base is 

 unity, it is quite evident that, if we lay 

 off a base line 10 in. long, the altitude 

 will be ID ins. x .5625, or 5.625 ins., as 

 in Fig. I. 



Again, suppose it is required to 

 determine the angle b, in Fig. 2. 



Lay off on — x, or — x produced, 

 — /, equal to 10 in. and from / erect a 

 perpendicular / — 5 of any convenient 

 length. Extend — y, if necessary to 

 intersect / — 5 at u. 



For reasons previously explained, t — u 

 is ten times the natural tangent of 

 angle b. Therefore, if t — u is measured 

 and found to be 4.78 in., the tangent of 

 b will naturally be 4.78-Mo, or .478 = 

 25° — 33'. — Harry B. Wrigley. 



Bracing the Back of a Long File 

 for Level Work 



THE drawing shows a method of 

 trussing a long file so as to make 

 the teeth on the cutting face more 

 effective, and also to produce a slightly 



A truss rod placed on a long file to brace 

 it and make the cutting more effective 



convex surface, enabling the file to be 

 used for broad work without rounding. 



A block is placed near the center of the 

 file, over which a 3<^-in. or ^/le-in. rod 

 passes. This has a hook to engage the 

 file at the forward end, and passes 

 through a hole drilled in the handle, 

 where a nut is run on the threaded end. 

 By tightening this nut the file may be 

 sprung slightly convex, increasing its 

 usefulness for certain classes of work. 



I 



Holding Reference Drawings on a 

 Drawing Board Conveniently 



T is quite necessary to have on the 



drawing board at times a bound 

 bunch of blue prints for reference and 

 they cannot be kept in place very well on 

 a slanting surface. To facilitate their 

 easy handling and hold them. in place 

 the arrangement shown was constructed. 



It consists of a wood cylinder A about 

 I >^ in. in diameter and about 24 in. long, 



Clamp arrangement to hold a bundle of 

 reference blue prints on a drawing board 



supported by two ^-in. bent rods which 

 are fastened to the bar B, the latter 

 being supported by two brackets C 

 fastened to the bottom side of the 

 drawing board. 



A rod D is fastened to the bar B and 

 extended to a convenient point at the 

 front of the drawing board. The rod D 

 is held in position by the hook E, which 

 is double, the same as the brackets C. 

 To release the drawings unhook rod D 

 and move it downward. The rod D can 

 be bent so that when it is in the hook it 

 will bring the proper pressure on the 

 cylinder A to hold the drawings in 

 position on the board. 



