Jimmie Hindman,, age 11, of Houston, Texas, for his question:

How do they tell the tides in advance?

Tide tables are prepared. by the government. They predict the tides far any area hour by hour far a year or morn ahead. The fob of figuring is done by complicated machines called tide predictors.

To make its predictions, a tide predictor must keep accurate track of the moon. For tides are caused by the moons pull of gravity. Sometimes the sun adds its pull to that of the moon. So the tidy predictor must also keep track of the sun in relation to the moon and the earth.

The tricky machine must also know the geography of each tidal area. For the depth of the sea or river, the shape of the shorn or bay cause changes in the tide. The paths of the sun and. the moon are known in advance. By correlating this information with geographical information the clever tide predictor can for©cast the tides in any area months and even years ahead.