produce a whispering gallery. Why Do We Get a Bump Instead of a Dent When We Knock Our Heads? When you knock your head against a sharp corner, or if some one hits you on the head with anything with a sharp edge, you do receive a dent in your head, but it does not last. In other words, the head has one of the qualities of a rubber ball. You can press your finger against the sides of the rubber ball and push it in, but when you take your finger off the ball resumes its shape. Just so with your head--it resumes its shape after a blow. After doing this, however, a bump or lump is formed. I will endeavor to tell you how the bump is formed or rather what causes it to form. You cannot knock your head against anything that is harder than your head without causing some injury to the parts which received the bump. Now, what happens then is just what happens to any other part of your body when it is injured whether as a result of a bump, a cut or a bee or mosquito sting. As soon as the injury occurs the brain starts the “repair crew” to work. The result is that first a great supply of blood is rushed to the injured part with the result that the blood vessels are filled up and extended with blood. Certain parts of the blood cells find their way through the walls of the blood vessels at the part of the injury and other fluids from the body are piled up there, so to speak, to form a congestion. This “piling up or congestion” distends the skin and raises the bump. On the head where the layer of muscular structure is thinner and where there is less space between the bones of the skull and the outside skin, the bump will be larger and more noticeable, because a good deal of blood and other fluids are piled up in a comparatively small space, and so the skin gets pushed out further to accommodate this great congestion, whereas in other parts of the body the bump may be quite as large but not so noticeable. [Illustration: HOW MEN GO DOWN TO THE BOTTOM OF THE SEA PUTTING ON THE SUIT. Socks, trousers and shirt in one, and a copper breastplate.] [Illustration: PUTTING ON THE IRON-SOLED SHOES. They are purposely made heavy, to help the diver sink.] The Deep Sea Diver What Does the Bottom of the Sea Look Like?