Larry ZcWie, age 11, of Visalia, California, for his question:

From what point is sea level measured?

The surface of the seas as everyone knows is in a constant state of upheaval. The tides rise and fall twice every day. What's mores these tides rise higher in some bays and beaches than in others. This goes on in all the vast oceans which cover most of our globe. With such a changeable surfaces how in the world can we talk about a standard sea level? The fact is, the level of the; sea is estimated to be the half way point between high tide and low tide. By and large it works out to be more or less the same all over the: earth.

This job of measuring sea level is not done with a ruler. It is estimated from :sir pressure, the weight of air pressing down upon a square inch. Above this square inch. A tall column of air stretches up to the top of the atmosphere. The higher we go, the less air there is to press down and the lower the air pressure. At sea level, the weight of air pressure is 14.7 pounds upon every square inch.