Returning to the _Natural Questions_, we may note that, like the _Problems_ of Aristotle, they vary from such crude queries as might occur to any curious person without scientific training to others that imply some previous theory or knowledge. A list of some of them will illustrate the scope of the scientific curiosity of the time. When one tree is grafted upon another, why is all the fruit of the nature of the grafted portion? Why do some brutes ruminate; why are some animals without stomachs; and why do some which drink make no water? Why do men grow bald in front? Why do some animals see better in the night than in the day and why can a man standing in the dark see objects that are in the light, while a man standing in the light cannot see objects that are in the dark? Why are the fingers of the human hand of unequal length and the palm hollow? Why don’t babies walk as soon as they are born, and why are they at first nourished upon milk, and why doesn’t milk agree equally with old and young? Why do we fear dead bodies?[64] A number of questions are devoted to each of the topics, vision, hearing, and heat, while the senses of taste, smell, and touch are dismissed in a single question and answer.[65] [Sidenote: Theory of sound.] The discussion of sound and vision may be noted more fully. The nephew has already learned from his Boethius something similar to the wave theory of sound. He states that when the air has been formed by the mouth of the speaker and impelled by the tongue, it impresses the same form upon that which is next to it, and that this process is repeated over and over just as concentric circles are formed when a stone is thrown into water.[66] Vitruvius had given the same explanation in discussing the acoustics of a theater.[67] But when the nephew asks his uncle how the voice can penetrate an iron wall, Adelard replies that every metal body, no matter how solid, has some pores through which the air can pass.[68] Thus he appears to regard air as the only substance which can transmit or conduct sound waves. His notion that air can pass through solids reminds one a little of the milder theory of Hero of Alexandria that heat and light consist of material particles which penetrate the interstices between the atoms composing air and water.[69] But it hardly seems as if Adelard could have derived _his_ notion from Hero, since the impermeability of metal vessels to air is a fundamental hypothesis in many of the devices of Hero’s _Pneumatics_. [Sidenote: Theory of vision.]