IOWA ACADEMY OF SCIENCES. 185 



interspaces. The board is revolved at the rate of about 

 one revolution in twenty seconds, by means of a small 

 electric motor and a system of pulleys. Both motor and 

 the pulleys have separate structures. The motor is placed 

 on a pier which is free from mechanical contact with the 

 building. The vibrations produced otherwise do not reach 

 the pen or board so that the machine is practically free 

 from all ordinary vibrations. 



The diameter of this drawing-board is about 2.10 meters. 

 The sheet of paper used was seventy-two inches wide. 

 [Since drawing-paper is a commercial article, the ancient 

 system is, of course, used.] The dividing-engine DE, figure 

 22, is only 45 cm. long. Now we should have a screw of more 

 than 100 cm. in length. In order to get around this obstacle 

 the following method was adopted: the gas-pipe, A B, 

 was passed through two supports, K B, one at each end of 

 the drawing-table. They were provided with screws, K, 

 so that the pipe could be firmly fastened and not moved 

 backward or forward. To H G, the movable bed of the 

 engine, a solid iron bar, G, having the same width as the 

 gas-pipe, is fastened. To this, in turn, are fastened three 

 flat brass bars seen at F. A visitor once remarked that 

 they reminded him of the inclined stacks of an ocean 

 steamer, but you may rest assured that the artistic effect 

 was not thought of during the construction of the appa- 

 ratus. When mechanical perfection is impossible it is 

 well to provide for the imperfections that are detrimental. 

 No attempt was made to drill a hole through which the 

 bolts would exactly fit. When the operator desires to 

 fasten the pipe to the engine he first forces the bars back 

 as far as possible and fastens the two lower bolts. Then 

 the upper bolt is fastened and thus the pipe is firmly 

 clamped to the bed of the engine. In order that the 

 operator ma> make measurements and watch the actions 

 of the pen there is mounted, at I, a Frauenhofer telescope, 

 CF. It is shown a little higher than the gas-pipe but 

 when in operation it is much nearer the same elevation. 

 From A to K are shown in part, the lever which raises the 

 pen and parallel to it the capillary tube for conveying the 



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