DRAWINGS . 163 



The chief instruments for mechanical 

 drawing are the rule, square, triangle, and 

 compass. A cheap kind of compass is 

 shown in the illustration. The rule is 

 used for making straight lines, the .square 

 for forming right angles, triangle for 

 obtaining different slants or angles, and 

 the compass for drawing circles or parts 

 of a circle. All that you need in your 

 science work is a rule and a compass. 

 The longest distance across a circle is its 

 diameter and the distance from the cen- 

 ter of the circle to the outside of the circle 

 is called the radius. 



If you wish to make copies of your 

 drawings the following experiment shows 

 one method. 



Experiment 68. Blue Prints from Tracings. 



Apparatus: Piece of window glass a little larger 

 than the drawing which is to be copied, thumb tacks. 



Materials: Tracing paper, blue-print paper. 



a. Make the drawing upon any kind of paper and 

 then cover it with a piece of tracing paper, fastening it 

 with thumb tacks. Trace the drawing with a very soft 

 pencil so that the line will be very dark. Use the rule 

 and compass when tracing just as you would in the first 

 drawing. After the tracing is made, place a piece of 

 blue-print paper upon a board or book and lay the trac- 

 ing upon it, right side up. Cover both with the piece 

 of window glass and expose to the sun until the blue- 

 print paper has become a bronze color. See Experiment 



