47. Explain some simple machine.
48. A new application of electricity.
49. Weather forecasting.
50. Scientific or practical value of polar expeditions.
51. Changes of the tide.
52. An eclipse.
53. The principle of some such appliance as the thermometer, the barometer, the microscope, the air-brake, the block signal.
54. Developing a negative.
55. How the player piano is operated.