~3. Sound and the Vibrations which it Causes.~--All sounds are made by jars or vibrations of objects. Sounds cause objects to vibrate or tremble. A loud sound sometimes jars a whole house, while other sounds are so gentle and soft that we cannot feel them in the same way that we feel loud sounds. But Nature has made for us an ingenious organ by means of which we can feel these very fine vibrations as well as loud ones. We call this organ the _ear_. ~4. The Ear.~--The part of the ear which we can see is shaped somewhat like a trumpet. The small opening near the middle of the ear leads into a _canal_ or tube which extends into the head about an inch. At the inner end there is a curious little chamber. This is called the _drum_ of the ear, because between it and the canal of the ear there is stretched a thin membrane like the head of a drum. The ear-drum is also called the _middle ear_. [Illustration: THE EAR.] ~5. Bones of the Ear.~--Within the drum of the ear there are three curious little bones which are joined together so as to make a complete chain, reaching from the drum-head to the other side of the drum. The last bone fits into a little hole which leads into another curious chamber. This chamber, which is called the _inner ear_, is filled with fluid, and in this fluid the nerve of hearing is spread out. A part of the inner ear looks very much like a snail shell. [Illustration: THE INSIDE OF THE EAR.] ~6. How we Hear.~--Scratch with a pin upon one end of a long wooden pole. Have some one listen with the ear placed close against the other end of the pole. He will tell you that he hears the scratching of the pin very plainly. This is because the scratching jars the ear and especially the drum-head, which vibrates just as the head of a drum does when it is beaten with a drum-stick. When the drum-head vibrates it moves the bones of the ear, and these carry the vibration to the nerves of hearing in the inner chamber. We hear all sounds in the same way, only most sounds come to the ear through the air. The snail-shell of the inner part of the ear hears musical sounds. The rest of the inner ear hears ordinary sounds or noises. ~7. How to Keep the Ears Healthy.~--The ears are very delicate organs and must be carefully treated. The following things about the care of the ears should never be forgotten: (1.) Never use a pin, toothpick, or any other sharp instrument to clean out the ear. There is great danger that the drum-head will be torn, and thus the hearing will be injured. Neither is it ever necessary to use an ear-spoon to remove the wax. Working at the ear causes more wax to form. (2.) Do not allow cold water to enter the ear or a cold wind to blow directly into it. (3.) If anything accidentally gets into the ear, do not work at it, but hold the head over to one side while water is made to run in from a syringe. If an insect has gone into the ear, pour in a little oil. This will kill the insect or make it come out.