As we have seen, the "lagoon" of the reef flat on the southeast side of Maër Island is shallow, being only about 18 inches deep at the lowest tide, although covered by about 8 feet of water at high tide. When the low tide falls at the hottest part of the day, at about 3 o'clock in the afternoon, the water of the reef flat is several degrees warmer than the air, but in the early morning before sunrise, the water is always colder than the air. This shows that the lagoon water derives most of its heat during the day from direct solar radiation, and at night the surface of the water radiates heat into outer space and thus becomes colder than the air. It has been commonly supposed that the temperature range of ocean water is less than that of the air, but this is evidently not the case in shallow lagoons in the tropics where the range in air temperature is slight. Indeed, during five weeks in September and October at the Murray Islands, the difference in air temperature between the hottest day and coolest night was only 10° F., the hottest being 86° and the coolest 76°; but during the same time the water over the southeast reef-flat
