That water is able to exert pressure in all directions may be proved by means of a small bladder attached to the end of a tube, as in the toy which is blown up and makes a squeal on letting the air escape. If, instead of letting the air escape, the tube is stopped up, and the bladder attached to a long rod and gradually lowered into a tank of water, the bladder is observed to become smaller and smaller as it gets lower down in the water. At the same time, it remains as round as it was originally, thus showing that the pressure of the water is exerted in every direction, and not on the top, bottom, or sides only. Another way of showing that water exerts a pressure is by tying a sheet of thin india-rubber over the end of a long and wide tube of glass, and lowering the end into the water. The india-rubber is seen to bend into the tube more and more as it is lowered farther and farther into the water. This shows that the pressure is in proportion to the depth. The pressure is not altered by putting any solid object into the water, so long as it does not change the level of the water. The pressure on the bottom of the vessel is the same so long as the level of the water remains constant. If the bottom of the tank is weak in any particular place, we can protect that place by laying a plate of iron across it, so long as there is no water between the iron and the weak place. For if there is any water between the bottom of the tank and the iron plate, that water hands on the pressure just as well as when the iron is not there at all. If instead of an iron plate, we put a solid block of iron suspended from above, the pressure on the bottom of the tank remains the same. The pressure simply depends upon the height of the surface of the water above the bottom of the vessel, and is not changed when the surface is made either very much larger or very much smaller. This curious fact was discovered by Stevin. He was Inspector of Dikes in Holland, and had a great deal to do with the pressure exerted by water upon the dikes which keep the sea away from the low-lying Netherlands. It was his business to make sure that the pressure of the water was not sufficient to break through the dikes.