IOWA ACADEMY OF SCIENCES 187 



the middle circle. It reads from .33^ to .66§, not so far as 

 .85. Turn the screw another complete revolution and 

 look at the divided head; use the inner circle this time 

 It reads from .GGf to 1. 00. The pointer on H is at the 

 second unmarked scale, but if you turn till the divided 

 head is set at 1. 00 you will find the pointer on H at 12. 00. 

 Ever}^ third division on H is marked in succession. There 

 are three spaces between 11. 00 and 12. 00. They corre- 

 spond to three turns of the screw. The two unmarked 

 divisions show which circle, on the divided head, to use. 



The two pointers are used to allow for the width of the 

 pen. The one at the right was used for the odd-numbered 

 circles. Suppose the position of one of these circles were 

 the lower margin of E, figure 24. If we had only this one 

 pointer your next line would be at I, say. But, this is too 

 far in a light zone by just the width of the line drawn by 

 the pen. The pointer at the left is set so that, turning the 

 head from the one on the right in the direction indicated, 

 will move the pen forward just the width of the line it 

 draws. The operator then works by the following method: 

 he draws the line E by setting the head on the right 

 pointer. For the next line he again turns until the proper 

 figure appears at this pointer but instead of drawing I he 

 turn and sets by the left pointer. He then draws F. The 

 process is quite simple and the operator becomes mechan- 

 ical in his actions as the work proceeds. 



There is one more point to consider before closing: 

 When the work is done with the beam compass, a small 

 conical hole in a tack serves as the center for all the 

 circles. We cannot do that with the present machine. In 

 order to have the pen pass through the axis of the ^^raw- 

 ing-table, or through the center of the circle it draws, the 

 table is firmly clamped and the pen moved over the paper. 

 It traces a long straight line. Now the board is revolved 

 through 180 degrees, firmly clamped, and the pen moved 

 over the surface as before. If the two lines coincide, the 

 pen passes through the center; if not, it can hd adjusted 

 until it does. 



