The invention of the aplanetic projector, due to Col. Mangin, was a clever means of overcoming difficulties, practically insurmountable, that were inseparable from the construction of parabolic mirrors; this contributed chiefly to the success of MM. Sautter, Lemonnier & Co. in this direction. The firm has produced more than 1,500 of these apparatus, representing a value of nearly L500,000, for the French and other governments. Besides the great projector, which forms the central and crowning object of the exhibit of MM. Sautter, Lemonnier & Co. in the machinery hall, the firm exhibits a projector 90 centimeters in diameter mounted on a crane traveling on wheels, in the pavilion of the War Department. The lamp used for this apparatus has a luminous value of 6,000 carcels, with a current of 100 amperes; the amplifying power of the mirror is 2,025, which gives an intensity of ten millions to twelve millions of carcels to the beam. Projectors used for field work are mounted on a portable carriage, which also contains the electric generator and the motor driving it. [Illustration: MILITARY PORTABLE ELECTRIC LIGHT AT THE PARIS EXHIBITION.] It consists of a tubular boiler (Dion, Bouton & Trepardoux system). This generator is easily taken to pieces, cleaned, and repaired, and steam can be raised to working pressure in 20 minutes. The mechanical and electrical part of the apparatus consists of a Parsons turbo-motor, of which MM. Sautter, Lemonnier & Co. possess the license in France for application to military and naval purposes. The speed of the motor is 9,000 revolutions per minute, and the dynamo is driven direct from it; at this speed it gives a current of 100 amperes with and from 55 to 70 volts; the intensity of the light is from 5,500 to 6,000 carcels. The carriage upon which the whole of this apparatus is mounted is carried on four wheels, made of wood with gun metal mountings. These are more easy to repair when in service than if they were wholly of iron. The weight of the carriage is three tons.--_Engineering._ * * * * * ELECTRIC MOTOR FOR ALTERNATING CURRENTS. Prof. Galileo Ferraris, of Turin, who has carefully studied alternating currents and secondary transformers, has constructed a little motor based upon an entirely new principle, which is as follows: If we take two inductive fields developed by two bobbins, the axes of which cut each other at right angles, and a pole placed at the vertex of the angle, this pole will be subjected to the simultaneous action of the two bobbins, and the resultant of the magnetic actions will be represented in magnitude and direction by the diagonal of the parallelogram, two consecutive sides of which have for their length the intensity of the two fields, and for their direction the axes of the two bobbins.