The Gyroscope exemplifies very clearly the persistence with which a body undergoing a movement of rotation maintains itself in the plane of its rotation in spite of gravitation. It shows also the result of the combined action of two forces tending to produce rotations round two axes which are separate, but situated in the same plane. The rotation of the wheel round its axis, produced, in the present instance, by the electro-magnet, and the tendency of the wheel to fall or turn in a vertical plane, parallel to its axis, produce, as a result, the rotation of the entire instrument round a new axis which coincides with the column. PEIFFER’S ELECTROPHORUS. It will now perhaps interest our readers to describe a charming little plaything which is a great favourite with children, and which has the incontestable merit of early initiating them into all the principal phenomena of the statics of electricity, and teaching them the science of physics in an amusing form. It is a small electrophorus invented by M. J. Peiffer, and reduced to such a point of simplicity, that it consists merely of a thin plate of ebonite, about the size of a large sheet of letter paper. The tinned wooden disc of the electrophorus which is found described in most treatises on physics, is replaced by a small sheet of tin, about the size of a playing-card, fastened on to the surface of the ebonite. The ebonite electrophorus produces electricity with remarkable facility. It must be placed flat on a wooden table, and thoroughly rubbed with the hand; if it is then lifted, and the sheet of tin lightly touched, a spark is elicited from ¼ inch to ½ inch in length. The electrophorus is completed by the addition of a number of small accessories in the shape of small dolls made of elder-wood, which exhibit in a very amusing manner the phenomena of attraction and repulsion. After the board has been charged with electricity, place the three little figures on the sheet of tin, and lift up the apparatus, so as to isolate it from any support. You will then see one little doll extending its arms, another with its silky hair standing on end, and a third, lighter than the others, leaping like a clown, and displacing as he does so the two small balls of elder-wood which have been placed on each side of him. We have given an illustration of the three figures performing at once (fig. 278), but they are generally used separately. M. Peiffer has indeed collected every known accessory for an electric machine, such as Geissler’s tube, the electric carillon, etc. These different experiments are all reduced to their simplest form, and, with their appliances, are all contained in a cardboard box. They are placed beside the electrophorus, which thus takes the place of an unwieldy electric machine. M. J. Peiffer accompanies this little portable cabinet with an exhaustive pamphlet, which is a valuable guide to the young physicist in studying the first principles of electricity.