782 



Popular Science Monthly 



through the bearing, collars being used to 

 hold it in place. The cross-arm is bolted 

 at the end of the shaft, care being taken to 

 screw the nuts up tightly so that the arm 

 will not slip on the shaft. The bevel 

 gears and pulley may be fitted, using an 

 ordinary shaft hanger next to the gear. 

 The other bearing may be made by using 

 a pipe nipple filled with babbitt metal and 

 bored to fit the shaft. It is then fastened 

 with screws to a rafter. The dimensions 

 given are for a motor of small power, but 

 they may be increased proportionately 

 for a higher powered motor. The motor 

 will always revolve in the same direction, 

 no matter from which point the wind 

 may be. 



Making an Adding and Subtracting 

 Machine of Cardboard 



IF our brains performed arithmetical 

 labors in the same way that calculating 

 machines do their work we should cer- 

 tainly have wheels in our heads, for cir- 

 cular motion is the basis of every practical 

 calculating device. Since our system of 

 numbers has ten for a basis almost all 

 the engaging wheels of computing ma- 

 chines have teeth that are ten or a 

 multiple of ten in number for convenience. 



The first 

 step in the 

 construction 

 of the adding 

 machine here- 

 in described is 

 to divide the 

 circumference 

 of a circle into 

 ten equal 

 parts. There 

 are scientific 

 ways of doing 

 it, but trial 

 measurements 

 with a pair of 

 dividers on the 

 circumference 

 will soon pro- 

 duce a close 

 approxima- 

 tion. To con- 

 struct the machine you will need a smooth 

 board 5 in. long, 4 in. wide and ^ in. 

 thick ; some heavy cardboard — the stouter 

 the better — for the two number wheels; 



Numbered cardboard wheels 

 for the adding machine 



two flat-headed wire brads for axles, 

 and a wire nail about 1 J-^ in. in length. 



To make the lower wheel of the ma- 

 chine draw a circle on the cardboard 2 in. 

 in diameter, then draw two tangents 

 that meet at a point beyond the circle. 

 Divide the circumference into ten equal 

 parts and number in black ink the division 

 points as shown. Following the lines 

 of the tangents with scissors cut out the 

 pear-shaped figure, and with a sharp 

 knife make a small triangular opening 

 in the V-shaped projection. 



The other wheel of the machine is also 

 2 in. in diameter and the circumference 

 is divided into ten equal parts. Describe 

 a concentric circle about 34 i^- inside of 

 the outer circumference, then carefully 

 cut the teeth as shown. To do this with 

 precision you should also divide the 

 inner circle into ten equal points and 

 make marks midway between the division 

 marks of the outer circle. Using these 

 marks for guides you will have no diffi- 

 culty in cutting the teeth accurately. 

 Number the second wheel in ink from 

 to 9 inclusive — a number on each tooth. 



The machine is now ready to set up. 

 Fasten the cogwheel first. Place it on 

 the board in such a position that the 

 teeth do not overlap the upper edge and 

 fasten it by one of the brads driven 

 through the center, drawing it well down 

 against the pasteboard, but not too tight 

 to prevent it from turning easily. 



With a pin for a temporary axle, de- 

 termine the proper position for the lower 

 wheel. It should be such that when the 

 wheel is turned the projecting point shall 

 engage the teeth of the upper wheel, but 

 will permit them to pass without cramp- 

 ing. When the position is correct, drive 

 a brad through the center to make all 

 parts secure. 



• Mark the board with the numbers 

 shown. Use a soft pencil and be guided 

 by the numbers on the lower wheel. 

 Draw a pencil guide line between the 

 two wheels so that it will appear through 

 the triangular, opening. In addition the 

 small arrows, one on the point of the 

 lower wheel and ' the other on the cog 

 number 9 of the upper wheel are drawn. 

 Make deep indentations on the lower 

 wheel on the inside of each of the num- 

 bers with a rather dull knife. These 

 serve as a holding place for the point 



