This section is from the book "Practical Cooking And Serving", by Janet McKenzie Hill. Also available from Amazon: Practical Cooking and Serving: A Complete Manual of How to Select, Prepare, and Serve Food .
When water is heated the air in it expands (bubbles seen on the bottom of the dish) rises and passes off. The heat now begins to change the water into an invisible gas, forming bubbles which are larger than the air bubbles. As they come near the surface they are cooled and burst, changing back into water. This bubbling below the surface is called simmering. The temperature of such water is about 185° Fahr. As the water grows hotter, part of the bubbles reach the surface, and there give off puffs of steam. The temperature is now 212° Fahr. By increasing the heat the water may be made to boil furiously, but the temperature will not rise higher; the extra heat is all used in turning the water into steam.
One of the first essentials to proper cookery is the ability to distinguish between simmering and boiling water.