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154 a little to the right, or, to put it in another way, a close threaded spiral is described round the drum. Now we shall irritate the nerve by an opening and a closing shock from the secondary coil of our induction machine, each shock coming always at the end of an equal interval of time. This we can arrange by attaching a wire to the axle of the cylinder, so that it stands out at right angles to the axle, and as the axle revolves, the wire dips into this trough containing mercury, thus closing the current of the primary coil of the induction machine, and the next instant the wire comes out of the mercury, thus opening the current of the primary coil. As you know, this will secure a shock from the secondary coil each time the wire dips into and comes out of the mercury. The current is led from the battery, through the primary coil, thence to the cylinder, thence through the wire into the mercury, thence back to the battery. Now notice the beautiful curve. It shows, first, the gradual lengthening of the period of latent stimulation, as indicated by each contraction beginning a little later than the one before it. You observe the gradual slope of the line joining the