However, between twelve and fifteen, after the demands of the boy’s physical activities and of his senses have somewhat abated, there comes “an interval when his faculties and powers are greater than his desires,” when he displays an insistent curiosity concerning natural phenomena and a constant appetite for rational knowledge. This period, which is dealt with in his third book, Rousseau declares to be intended by nature itself as the time for instruction. But as not much can be learned within three years, the boy is to study only those subjects which are useful and not incomprehensible and misleading, and so is limited to the natural sciences. Later in this third book, in order that Emile may informally learn the interdependence of men and may himself become economically independent, Rousseau adds industrial experience and the acquisition of cabinet-making to his training. The most effective method of instruction, Rousseau holds, comes through appealing to the curiosity and interest in investigation, which are so prominent in the boy at this time. He contrasts the current methods of teaching astronomy and geography by means of globes, maps, and other misleading representations, with the more natural plan of stimulating inquiry through observing the sun when rising and setting during the different seasons, and through problems concerning the topography of the neighborhood. Emile is taught to appreciate the value of these subjects by being lost in the forest, and endeavoring to find a way out. He learns the elements of electricity through meeting with a juggler, who attracts an artificial duck by means of a concealed magnet. He similarly discovers through experience the effect of cold and heat upon solids and liquids, and so comes to understand the thermometer and other instruments. Hence Rousseau feels that all knowledge of real value may be acquired most clearly and naturally without the use of rivalry or textbooks. But he finds an exception to this irrational method in one book, _Robinson Crusoe_, “where all the natural needs of man are exhibited in a manner obvious to the mind of a child, and where the means of providing for these needs are successively developed with the same facility.” [Sidenote: In youth, sex interests, as basis of moral and social training.]