The main or platform pendulum resembles in this case that of the Rectilinear Harmonograph, the card platform being above the point of suspension. Value of the Harmonograph. A small portable harmonograph will be found to be a good means of entertaining friends at home or elsewhere.

The screw collar for holding the valve in place had a little brass disc soldered to the outside, and this disc was bored centrally for the threads to pass through. The edges of the hole had been rounded off carefully to prevent fraying of the threads. The weights of the lower pendulum are several in number, ranging from l lb. to 3 lbs. Working the Harmonograph. A preliminary remark is needed here.

The tuning of a harmonograph with independent pendulums is a simple matter. It is merely necessary to move weights up or down until the respective numbers of swings per minute bear to one another the ratio required.

Don't ask people with weak lungs to try experiments with the apparatus described in this chapter. Have you ever heard of the harmonograph? If not, or if at the most you have very hazy ideas as to what it is, let me explain. It is an instrument for recording on paper, or on some other suitable surface, the figures described by two or more pendulums acting in concert.

This type of harmonograph, if made of convenient size, has its limitations, as it is difficult to get as high a harmonic as 1:2, or the octave with it, owing to the fact that one pendulum must in this case be very much shorter than the other, and therefore is very sensitive to the effects of friction. Hamonograms illustrating the ratio 1:3.

We have now gone far enough to be able to point out why the harmonograph is so called. In the case just mentioned the period rates of A and B are as 2: 3. Now, if the note C on the piano be struck the strings give a certain note, because they vibrate a certain number of times per second.

Before closing this chapter I should like to acknowledge the kind assistance given me by Mr. C. E. Benham, who has made a long and careful study of the harmonograph. This useful little article can be constructed in a couple of hours by a handy person. The top of A is grooved to allow a match to rest on it. The only materials required are a cigar-box, some pins, and a supply of glue.

The card on which the figure is to be traced, and the weights, are placed on a platform at the bottom of this pendulum. A second and shorter free pendulum, known as the "deflector," hung from the bottom of the first. This form of harmonograph gives figures of infinite variety and of extreme beauty and complexity.

They are beautiful and of infinite variety, and one advantage attaching to this form of harmonograph is, that the same figure can be reproduced exactly an indefinite number of times by releasing the pendulums from the same points.

But the interest of the harmonograph centres round the fact that the periods of the pendulums can be tuned to one another. Thus, if A be set to swing twice while B swings three times, an entirely new series of figures results; and the variety is further increased by altering the respective amplitudes of swing and phase of the pendulums.