Rambam - 1 Chapter a Day

The following laws apply when a support is firmly joined to the earth or nailed down. If it is sound and strong and a zav banged upon it and when doing so, a k'li, food, or liquids that were resting on it were moved or caused to fall from the power of his blow, they are pure. If the support was not sound or strong, but shaky and a zav banged upon it and caused keilim, food, or liquids to fall or move from the power of his blow, they are impure. It is as if he moved them, because they fell as a result of his power.

This is the general principle: Whatever falls as a direct result of the power of the zav's movement is impure. Anything that falls due to the trembling of the support is pure.

What is implied? If he knocked on a tree or a booth that were not sturdy, on an Egyptian ladder that was not affixed with nails, on a ramp, a beam, or a door that was not firmly held in place with mortar or he knocked on a door-bolt, a lock, or an oar, and when he knocked upon these entities, he caused food, liquids, or keilim to fall - or even caused them to shake without falling - they are impure. If, however, he knocked on a tree or a booth that was strong, or an Egyptian ladder that was nailed down, or a ramp, a beam, or a door that was held in place with mortar, a central building beam or an oven, and when he knocked on that entity, keilim, foods, and liquids on it were moved - and even if they fell - they remain pure. Similar laws apply in all analogous situations.

When there was a loaf of bread attached to the wall of an oven and a zav knocked on the oven and caused the loaf to fall, it is pure, even if the loaf is terumah. If a shard from the oven is clinging to the loaf that fell, it is impure, even if the loaf is ordinary food.

A surface on which one lies or rides that has contracted midras impurity does not impart impurity to keilim, foods, or liquids, unless it touches them. If, however, the keilim, foods, or liquids were above the impure midras or the impure midras was above them, as long as they do not touch it, even if there is only a paper separating between them, they are pure. Similarly, if an object on which one sits caused keilim, foods, or liquids to move, they are pure.

When a zav and a pure person were sitting in a small boat that could tip due to the movement of one person, on a dinghy, or riding an animal together, even if their clothes are not touching each other, the clothes of the pure person are considered to have contracted midras impurity. This impurity comes as a result of a doubt; perhaps the zav stepped upon them. Similarly, if a woman in the nidah state sat together with a woman who is pure on a bed, the other woman's clothes are impure because of midras impurity, with the exception of the cap on her head. Similarly, when a zav was in a small boat, clothes on top of the masthead are pure.

When one pushes into a mill in which a gentile, a zav, or the like is located, his clothes contract midras impurity.

With regard to which type of mill was this said? One that shakes the ground to the extent that it will raise the person standing there and force him from his place.

It is obvious that this impurity was decreed due to a doubt: perhaps the impure person was displaced and stepped on the clothes of the pure person.