The wonderful qualities of percussion, which must be explained and demonstrated at the end, require the structure of solid bodies to be known. This will be dealt with in the present chapter. The largest and principal part of the bodies subjected to our senses is fluid. Such are air, water, fire, quicksilver and countless others. These obviously have neither consistency nor hardness. They can be divided and penetrated by any body. Although the primary components of these fluids are absolutely hard and inelastic as seems logical, nevertheless their mass and their aggregate which compose a fluid body can be divided into particles which can in no way run and move. They offer, however, some resistance, not against division, but against movements in so far as they are heavy. They oppose any compressing body introduced underneath them in so far as they must be raised upwards from their position. They resist this raising by the intrinsic force of their gravity. They also resist sideways expulsion in so far as they must be moved from their immobility and equilibrium, and push the surrounding parts of the fluid to yield to themselves and to the impelling body penetrating beneath them. These particles while insinuating and moving between the other immobile particles must also rub against and overcome the resistances of their lateral protuberances. In such fluid bodies many heterogeneous particles of irregular shape are mixed such as particles of earth which are mixed with air and water. These particles increase the resistance against lateral movement. There are also soft bodies like mud, honey, wax and hot metals. These also resist in so far as they take part in the nature of a fluid and, moreover, inasmuch as they contain bigger and rougher particles which, therefore, are more difficult to extricate from each other whenever they have to move between the other immobile particles and push forwards particles which they meet and insinuate them between adjacent particles. All these causes increase their resistance whenever they must be impelled or divided and penetrated by a hard body. This difficulty is the greater the more the parts are tied together or entwined by their cohesion or pertinacity as occurs in metals which present with some softness since they yield to hammer blows. There are some cleavable bodies such as wood which is made of orientated fibres attached to each other and tied by some bond or cohesion. They thus resist division much more if their fibres must be cut than if they must be dissociated only. Finally there are flexible bodies which, like a machine, spontaneously straighten after being bent. These resist thanks to levers which are arranged parallel to each other over all the length of the flexible body. Although their particles are squeezed or stretched from each other, nevertheless they are not expelled completely from their position as occurs in soft bodies. Finally, there are bodies which are commonly considered as hard such as stones. Their resistance, however, is not insuperable. Indeed they can be divided and split. Some thus are friable. Others, constricted by more tenacious cohesion do not at all split into very small particles. Generally, however, one must admit in all solid bodies some compressibility thanks to which their parts not only bend but also contract, condense and dilate somewhat. This is deduced not only from their crushing, rupture, flattening, filing but also from the sound which they produce when struck by some motive impetus. Indeed a sound cannot be emitted without vibration of air and vibration in air can be generated only by repeated blows on a shaking and vibrating body. Thus we experience shaking not only in small bodies but also in huge buildings and even in the Alps. If indeed somebody lives in a deep cave in a mountain and the mountain is struck outside, the sound is perceived inside the cave. This obviously suggests that the huge mass of the mountain is somewhat agitated by the shaking. If, consequently, all bodies, even the hardest, appear to emit sounds after being struck, one must recognize that all the bodies which are commonly considered as hard bend and shake by trembling. But they can bend only if their parts are squeezed on one side and stretched on the other.