英文科學(xué)讀本 第五冊·Lesson 52 Currents

Lesson 52 Currents

I think you have seen this before, said Mr. Wilson, as he proceeded with an experiment at the opening of the next lesson. The boys watched carefully, and were soon eager and anxious to tell him all about it.

Well, Fred, he continued, "suppose you tell us what it is."

This is the experiment by which you showed us the heating of liquids by convection, sir, said Fred.

Exactly, replied Mr. Wilson, "But I am now using it for another purpose. Why did I put the little pieces of coloring matter into the water?"

The little blue particles show us the direction in which the water moves, sir.

Right, my lad. I am now going to talk to you about this moving water. The coloring matter shows us, as you say, streams flowing through the body of the water in the flask. These streams we call currents. You see it is the heat from the flame below which sets the water in motion, and causes it to flow in streams or currents through the flask. Let us now place the heat above the water, instead of below it. I suppose you wonder how I am going to do that. Look; I have here a large open glass trough filled with water, and into the water, as before, I will drop some little pieces of coloring matter. I have also in the fire, red-hot, the iron ball which we have already used for another purpose. Here it is; I will hold it by its chain just over one end of the trough.

The water immediately under the red-hot ball is heated, and of course expanded, thereby becoming lighter, bulk for bulk, than the body of the water around and below it, while some of it actually passes away by evaporation. This lighter water is forced along by the upward pressure of the denser liquid below it, and thus a stream or current is set up through the trough, the warm water flowing along the surface, the colder, denser stream along the bottom of the vessel. Now I want this red-hot ball to represent the sun, and this trough of water the ocean. I shall be able to make you understand readily enough how similar currents are set up in the ocean by the heat of the sun.

We have frequently, in our lessons on evaporation, spoken of the effect of the sun in heating the water of the ocean. In what part of the world should we expect to find this going on to the greatest extent? Your thoughts immediately travel, of course, to the great oceans in the tropics, and you are quite right. The immense power of the sun in these regions heats the water of the ocean in very much the same way as our red-hot ball heated the water just now. Some of it, we know, rises in masses of vapor to form clouds; that which is not evaporated is expanded with the heat, and becomes lighter, bulk for bulk, than the body of the ocean water. The denser water from below exerts a greater pressure upwards than this expanded, lighter water can exert downwards. The balance has been upset between them; the stronger force gets the mastery; the light, expanded, surface water gives way, forced onwards by the pressure of that from below. In this manner, surface streams or currents of warm water are set up from the equatorial or tropical seas towards the poles. But let us go a step further. The dense, cold water, which by its upward pressure forces these currents onward, itself comes from the lower depths of the ocean. Such immense masses of water drawn from below tend to destroy the equilibrium—to create a vacuum. To prevent this, other streams or currents set in from the poles towards the equatorial seas. The water of these polar currents is, of course, cold, dense, and heavy. They flow, as we should expect to find them, along the bed or bottom of the sea.

I have, thus far, spoken of these currents as flowing either from the equator to the poles, or from the poles to the equator. A chart of the ocean, however, would show us that they do not flow direct north and south. They would, if the earth were stationary, and if there were no great masses of land to intercept them. As it is, these currents are turned out of their course by the rotation of the earth on its axis, as well as by the irregular configuration of the land. The most important of these ocean currents is the great Equatorial Current, which commences off the west coast of Africa, crosses the Atlantic to the shores of South America, and after passing through the Caribbean Sea and the Gulf of Mexico, emerges once more into the Atlantic under the name of the Gulf Stream. It is veritably a warm river rushing through the Atlantic at the rate of about 50 miles a day. It skirts the eastern shores of North America, and at length disperses, some say in mid-ocean, others (and this is more generally believed) off the western shores of Europe.

I called it a warm ocean-river just now. When it leaves the Gulf of Mexico its average temperature is 81℉., and even at New York its temperature is about 75℉. You will perhaps form some idea of the immense volume of this ocean stream when I tell you that its depth off New York is not less than 100 fathoms, while at Florida it is probably twice that depth. It leaves the Gulf with a width of 32 miles, which has increased off Cape Hatteras to 75 miles. This immense volume of water then flows through the Atlantic, parting with its heat on its way, and it is generally believed to have a very marked and beneficial effect on the climate of Western Europe.

I have sketched a brief outline of this very important current, merely to give you some idea of its course, its immensity, its usefulness. We are chiefly concerned here, however, with the producing cause of the currents—not with the currents themselves. I shall be satisfied if you remember that, as heat is the cause by which winds are produced in the ocean of air, so heat is again the cause by which currents are produced in the ocean of water."