Many curious and interesting experiments may be performed by means of the machine, illustrating the general properties of electricity. For instance: a person standing on an insulated bench, that is, a bench with glass legs, or having the legs resting on glass, and having one hand on the conductor, can send sparks, with the other hand, to everything and everybody about. This illustrates communication of electricity by contact. A wooden head, covered with long hairs, when placed on the conductor, illustrates electrical repulsion, by the hairs standing on end. If the hand is held to the knob, sparks will pass from it in rapid succession, causing in the hand a sensation of pain. This is called an _electric shock_, and is caused by the electric fluid occasioning a sudden motion by the contraction of the muscles through which it passes. The force of the shock is in proportion to the power of the machine. What are the Muscles? Bundles of thin fleshy fibres, or threads, fastened to the bones of animals, the contraction and expansion of which move the bones or perform the organic functions of life. _Organic_, relating to organs or natural instruments by which some process is carried on. _Functions_, employments or offices of any part of the body. _Contraction_, drawing in or shortening. _Expansion_, extending or spreading out. What is Twilight? The light from the first dawning of day to the rising of the sun; and again between its setting and the last remains of day. Without twilight, the sun's light would appear at its rising, and disappear at its setting, instantaneously; and we should experience a sudden transition from the brightest sunshine to the profoundest obscurity. The duration of twilight is different in different climates; and in the same places it varies at different periods of the year. _Instantaneously_, done in an instant, in a moment's time. _Obscurity_, darkness, want of light. How is it produced? By the sun's refraction--that is, the variation of the rays of light from their direct course, occasioned by the difference of density in the atmosphere. _Variation_, change. _Density_, closeness of parts, compactness. What is the poetical name for the morning Twilight? Aurora, the goddess of the morning, and harbinger of the rising sun: whom poets and artists represent as drawn by white horses in a rose-colored chariot, unfolding with her rosy fingers the portals of the East, pouring reviving dew upon the earth, and re-animating plants and flowers. _Harbinger_, a forerunner. _Portals_, gates, doors of entrance. _Reanimating_, invigorating with new life. What remarkable phenomenon is afforded to the inhabitants of the polar regions?