Flow means the coming in of the water according to its nature and the laws of its motion. The ebb is the going out of the water, and rests on laws that are the reverse of those of the flow. With respect to the ebb and flow, the seas may be divided into three classes, the first of which comprehends those seas in which ebb and flow take place, and are apparent and evident; the other in which ebb and flow take place, but are not perceptible; and finally, there are seas in which there is no ebb and flow at all. Such seas are on volcanic ground, and if the ground is in volcanic action, the water is in a constant current to another sea, being increased in volume and swelled by the air that is originally in the earth, and thence communicated to the water. This is particularly frequent with seas that have an extensive coastline and many islands.
A variety of opinions have been ventured regarding the causes of the ebb and flow. Some ascribe them to the influence of the moon, for she, being congenial with water, makes it warm and expands it. They compare her influence with that of fire: if water is exposed to the influence of heat in a kettle, even if only one-half or two-thirds of the kettle is full, it will rise when it boils until it runs over, because its volume becomes apparently double as its weight is diminished, it being a law of heat to expand bodies and a law of cold to contract them. The bottom of the sea becomes warm, and by these means sweet water is produced in the earth and is changed into salt water and becomes warm, as happens in cisterns and wells. When the water is warm, it expands; and when it is expanded, it is increased in volume; and when its volume is great, every particle pushes the particle next to itself, and so it raises the level as it rises from the bottom, because it requires more space. The full moon communicates a great deal of heat to the atmosphere; hence the water increases in volume. This is called the monthly tide.
Others say that if the ebb and flow is the same phenomenon as the expansion of water in a kettle under the influence of fire, making the water rise, the sea will, after it has been removed from the bottom of its basin, go according to its nature to the deepest places of the earth, and so it will return to its former place, just as the water that boils in a cauldron returns as quickly to the bottom of the vessel as it is displaced by the particles of the fire.
The sun is the warmest body; if the sun was the cause of ebb and flow, the latter would begin with the rising of this luminary, and the former with its setting. They believe, then, that ebb and flow is caused by vapors, which are produced in the bowels of the earth and continue to be generated until they are discharged. This discharge pushes the water of the sea, and it remains in this state until the pressure from underneath is diminished; then the sea returns to the depths of its bed and the ebb succeeds.
Others say that the motion of the waters of the sea is not different from the vicissitudes of the temperaments in men. You may observe in choleric, sanguine, and other persons that their temperament is roused for a time but then is quiet again. In the same way, the sea rises by degrees, and when it has come to the greatest intenseness, it sinks by degrees.
From Meadows of Gold. Born in Baghdad around 893, al-Masudi began traveling when he was nineteen. Over the course of his travels—ranging from India to Palestine—he studied under a philologist at Basra, met a scholar who was translating the Torah in Tiberias, and studied Hellenistic ruins in Damascus. He interviewed soldiers, merchants, and officials in various countries for his research for this book. The editors of one translation note that “it was not until the Renaissance that European historians were able to work under the sort of conditions that Masudi took for granted.”
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