Page:Popular Science Monthly Volume 92.djvu/798

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��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

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