Jack Evans, age 10, of San Diego, California, for his question:

What causes the Gulf Stream?

In some places, you can sail a mile across the Mississippi    and dip down 100 feet. It carries four million tons of water on its long journey to the sea. The Gulf Stream carries four billion tons of much deeper, wider water    and the round trip may be 10,000 miles or more. Obviously the forces that run it must be extremely powerful.

Let's picture the Gulf Stream as an enormous water wheel, eddying around the North Atlantic. It speeds up and slows down, squeezes through narrowish channels and sprawls more than 1,000 miles wide. It carries 1,000 times more water than the Mississippi. If its curves could be remodeled, they could encircle North America. And, if all our power plants united, their efforts would be much too weak to cause this mighty Gulf Stream.

Without help, even the earth cannot provide the energy it needs. However, the starry sun sheds radiant energy that causes certain changes in the ocean and the atmosphere. And the spinning earth uses these changes to boost the Gulf Stream around and around its endless cycle.

Let's trace this fancy teamwork to the power station at the equator. Here the world's wide waist gets a generous quota of solar radiation. The tropical sun warms the tropical sea and the water warms the air. Heat causes air and water to expand. A ridge of warm, swollen water rises and forms the North Equatorial Current. Bubbles of warm light air rise aloft and the trade winds blow in to fill the airy vacancy.

At this point, the spinning earth imposes traffic rules that twist the trade winds and twirl the Gulf Stream on its way. Since the earth is a globe, the wide equator must rotate fastest to keep up with the rest of the map. This causes odd happenings. Between the equator and the poles, the winds and waters seem to lag behind. Actually the equator happens to be moving ahead faster.

In any case, north of the equator this causes the trade winds to veer toward the southwest and the North Equatorial Current sags toward the west. The stage is set to provide the power that drives the Gulf Stream. The strong, steady trades drive the equatorial current from Africa to America.

Once started on its way, other factors steer the mighty ocean current north and then east across the Atlantic, then southward down the coast of Africa. There it re¬joins the North Equatorial Current. This is where the sun and the spinning globe team up with the trade winds and masses of warm air and water. This teamwork causes the birth of the Gulf Stream and provides its energy. Land barriers help to steer it around its cycle.