Fig. 11.
Fig. 12.
A piece of thin parchment, A, Figs. 11 and 12, is stretched tightly across a circular hoop of wood. B C is a light lever turning on D. At the end, B, is a vertical needle-point touching the membrane, A, and at C is another needle-point, projecting horizontally and touching a smoked glass plate, E F. This glass plate is drawn along in the direction, H G, by clockwork, K. The end, B, of the lever is weighted so that it shall quickly follow the movements of the centre of the disc, A. These movements are transmitted and recorded on the glass plate, E F, by means of the lever and needle-point, C. Holes are cut in the side of the hoop to allow a free passage of air to the under side of the membrane. The apparatus was well tested beforehand by myself and others, to see that no shaking or jar on the table or support would interfere with the results. The line traced by the point, C, on the smoked glass was perfectly straight in spite of all our attempts to influence the lever by shaking the stand or stamping on the floor.
Fig. 13.
Experiment V.—Without having the object of the instrument explained to her, the lady was brought into the room and asked to place her fingers on the wooden stand at the points, L M, Fig. 11. I then placed my hands over hers to enable me to detect any conscious or unconscious movement on her part. Presently percussive noises were heard on the parchment, resembling the dropping of grains of sand on its surface. At each percussion a fragment of graphite which I had placed on the membrane was seen to be projected upwards about 1-50th of an inch, and the end, C, of the lever moved slightly up and down. Sometimes the sounds were as rapid as those from an induction-coil, whilst at others they were more than a second apart. Five or six tracings were taken, and in all cases a movement of the end, C, of the lever was seen to have occurred with each vibration of the membrane.
In some cases the lady's hands were not so near the membrane as L M, but were at N O, Fig. 12.
The accompanying figure 13 gives tracings taken from the plates used on these occasions.