Measure and record the temperature of water at various places -- top, sides, bottom, corners -- in a 5 gallon or larger aquarium filled with water. You will find that the temper- ature of the water is much the same throughout the container.

Along the same side of the aquarium but at opposite ends, tape two identical thermometers in such a way that the bulb of one is near the surface of the water and the bulb of the other is near the bottom. Now place a row of ice cubes at the end where the thermometer bulb is near the surface. Keep the cubes at least an inch from the thermometer.

Read the temperatures on both thermometers at one-minute intervals. The thermometer further away from the ice will soon show a lower temperature.

Try to explain why this happens. You may suggest that the cold water traveled downward and along the bottom. Check this explanation by putting several drops of food coloring just below the melting ice. (You will see a colored current of water traveling downward from the ice cubes and along the bottom of the aquarium.)

(Note: The greater the difference between the temperature of the ice and the water, the faster the current.)

In a similar way, temperature differences between equatorial and polar ocean water and between surface and subsurface water causes the water to circulate. (Equatorial waters average about 80 degrees F (27 degrees C); polar waters average about 30 degrees F (-2 degrees C). The temperature of ocean water also increases with depth, and ocean water is very cold at extreme depths.)

Another activity makes use of an aquarium heater. Attach a heater to one end of the aquarium, and arrange the bulbs of two identical thermometers so that the one nearer the heater is at the bottom of the aquarium, and the other is at the top. Predict what will happen, then turn on the heater and note the temperatures at one-minute intervals. (The thermo- meter further away from the heater will change first.)

The movement of the current can be seen by adding food coloring near the heater. (Note: If the heater is placed near the water surface, you will be able to identify thermal layers in the water; if the heater is placed near the bottom, you can identify movements or currents in the water.)

There are many possible variations of this experiment. For example, several thermometers and/or ice cubes can be placed at different levels and/or locations, and various water temperatures can be used.