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in this Institution in 1855, and who has taken a warm interest in the success of the present course. I put a weight on the scale-pan below the lever and irritate the muscle. Observe it can lift, as you hear by the tone of the bell, 5, 10, 20, 30, 40, 50, 100, 200, 250, 300, 350, 400, even up to 500 grammes. It can Fig. 12.—Essential part of frog interrupter used in experiment represented in Fig. 11. m, muscle; n, nerve; c, lever; when c is raised by the contracting muscle m, the contacts at x and y are broken; x is a platinum where dipping into mercury, and y is a contact between two platinised surfaces. The arrows near the wires connecting x y show direction of current. When contact is broken by lifting the lever c, the bell b (middle) in Fig. 11 rings. actually, by a sheer pull, move a mass one thousand times its own weight. Is not this a wonderful expenditure of mechanical energy?