I want you now to consider what is happening when two flat plates partly immersed in water are held close together. We have seen that the water rises between them. Those parts of these two plates, which have air between them and also air outside them (indicated by the letter _a_ in Fig. 11), are each of them pressed equally in opposite directions by the pressure of the air, and so these parts do not tend to approach or to recede from one another. These parts again which have water on each side of each of them (as indicated by the letter _c_) are equally pressed in opposite directions by the pressure of the water, and so these parts do not tend to approach or to recede from one another. But those parts of the plates (_b_) which have water between them and air outside would, you might think, be pushed apart by the water between them with a greater force than that which could be exerted by the air outside, and so you might be led to expect that on this account a pair of plates if free to move would separate at once. But such an idea though very natural is wrong, and for this reason. The water that is raised between the plates being above the general level must be under a less pressure, because, as every one knows, as you go down in water the pressure increases, and so as you go up the pressure must get less. The water then that is raised between the plates is under a less pressure than the air outside, and so on the whole the plates are pushed together. You can easily see that this is the case. I have two very light hollow glass beads such as are used to decorate a Christmas tree. These will float in water if one end is stopped with sealing-wax. These are both wetted by water, and so the water between them is slightly raised, for they act in the same way as the two plates, but not so powerfully. However, you will have no difficulty in seeing that the moment I leave them alone they rush together with considerable force. Now if you refer to the second figure in the diagram, which represents two plates which are neither of them wetted, I think you will see, without any explanation from me, that they should be pressed together, and this is made evident by experiment. Two other beads which have been dipped in paraffin wax so that they are neither of them wetted by water float up to one another again when separated as though they attracted each other just as the clean glass beads did. [Illustration: Fig. 11.]