Curiosities of Heat

by Lyman Beecher Tefft

“We have already spoken of the transfer of heat from the torrid to the temperate and frigid zones by the agency of winds and watery vapor. These carry heat chiefly in a latent condition. But great movements of heat take place in a sensible state. In this transfer of heat, also, water is the great carrier. The winds and vapor go freighted with latent heat above, and the waters and wind go freighted with sensible heat below. We will first examine the operation of the ocean currents.

“Not only do rivers run through the lands and hasten to the sea, but in the midst of the oceans rivers are flowing in comparison with which the Mississippi, the Amazon, and the Yang-tse-kiang are rippling brooklets. The earth is belted by these ocean streams traversing the seas. An ocean current, called the Gulf Stream, issues from the Gulf of Mexico between the Florida coast and the Bahama islands. It flows northward off the coast of the United States, gradually increasing in breadth and spreading over the Atlantic Ocean. It is deflected by the New England coast and the great shoals off Newfoundland, called the Grand Banks, or else by another current flowing southward from Baffin’s Bay, and strikes across the North Atlantic, bathing the shores of the British islands and reaching even to Iceland.

“The general outline of the ocean currents is this: issuing from the South Pacific, a current flowing eastward splits upon Cape Horn. The western portion, called Humboldt’s current, flows northward along the western coast of South America, and is swallowed up and lost in the great equatorial current of the Pacific. This is a broad current flowing westward and covering the entire space between the tropics. Striking upon the eastern shores of Asia, this equatorial current divides, one part flowing northward along the coast of Asia, the other finding its way through the many islands, sweeping across the Indian Ocean, and flowing down the eastern shore of Africa on each side of Madagascar. Doubling the Cape of Good Hope, the current continues in a north-westerly direction across the Atlantic. Striking upon Cape St. Roque, this current again divides; a part flows south and a part pours into the Caribbean Sea. From the Caribbean Sea it issues as the Gulf Stream, of which I have already spoken. This Gulf Stream impinges upon the western coast of Europe, and pours partly into the North Sea and partly flows south off the western coast of Africa, completing thus the circuit of the Atlantic. The currents of the Indian and of the great Southern Oceans are as yet very imperfectly understood. Of all the ocean streams the Gulf Stream is most famous and best understood. I shall therefore use this as an illustration of the agency of ocean currents in conveying heat and modifying climate.

“The waters of the Caribbean Sea are heated by the tropic sun to eighty-eight degrees. From these heated waters the Gulf Stream issues salter and warmer, and of a deeper blue, than the waters of the surrounding sea. Its greatest velocity as it issues from the gulf is a little more than three miles per hour. As it flows northward its velocity diminishes, its breadth becomes greater, and its depth less. It covers thus with its warm waters a broad belt of the Atlantic Ocean, and extends its influence to the most northern part of Europe. You can judge of the amount of heat which is removed from the tropics when I tell you that the unmeasured flood of the Gulf Stream would swallow up three thousand rivers like the Mississippi. This one ocean stream is many times greater than all the rivers of the world. We feel the warmth of the Gulf Stream with every wind that blows from the sea. To this the British isles owe their mild, moist climate and perennial greenness, and by its influence a winter in Iceland, upon the Arctic circle, is no more rigorous than a winter in Montreal, twenty-one degrees nearer the equator. But what is the Gulf Stream, though it be fifty fold greater than all the rivers of the world, in comparison with the whole sum of the ocean streams? Upper currents and under currents fill the sea. They meet the explorers of the sea everywhere. The navigator drops his measuring line, and finds it swept away and drawn out by unseen currents. All these movements of the waters are in favor of the equalization of temperature. The cooler waters of the frigid and temperate zones are mingled with the heated waters of the tropics and exchanged for the equatorial waters. The transfer of heat would not be greater if broad rivers of molten lava were flowing from the equator to the poles.

“Another agency for the transfer of heat is the movement of ice, and especially of icebergs.”

“Will you not tell us,” said Samuel, “how these ocean currents are produced? I can understand how winds are formed, but I do not see that these streams in the sea could be formed in the same way.”

“I designed to speak of this, but for the moment it had slipped from my mind: I am glad that you called my attention to it. I do not expect, however, to give a full and satisfactory account of their origin. If I should do this, I should succeed where every other man has failed. I shall not attempt a full explanation. By some means or other, the waters of the ocean are thrown out of equilibrium, and these currents are plainly an effort to restore the balance or equilibrium of the waters. Many influences and agencies conspire to disturb the equilibrium of the sea. The attractions of the sun and moon are constantly counteracting the attraction of the earth and lifting the waters, so to speak, above their natural level. The tides produced by these attractions of the sun and moon are the immediate cause of some of the minor local currents. The winds set the waters in motion, tending to pile them up in one place and leave the sea below its natural level at another. The effect of strong winds in piling up the waters, even upon our great lakes, is very considerable. A heavy east wind upon Lake Erie has been known to drive the waters toward the western end of the lake so much as to leave Niagara River above the falls almost dry. On the other hand, a heavy west wind drives the waters eastward, and produces almost a flood in the river. The influence of constant winds like the ‘trades’ acting upon an immense expanse of water must be very much greater. Unequal evaporation tends to destroy the balance of the waters. In the colder regions the evaporation is very little, while within the tropics it amounts to about half an inch daily, or fifteen feet per annum. The head of the Red Sea is two feet lower than its mouth on account of evaporation. This unequal evaporation causes also an unequal saltness, and consequently an unequal weight. The fresher and lighter water cannot balance an equal bulk of salter and heavier water. When once currents are started the revolution of the earth upon its axis would affect them, just as the rotation of the earth affects the trade-winds. Now, all these various agencies, and perhaps many others, combine their influence to destroy the equilibrium of the waters of the ocean. They unite and interweave their influence in a thousand ways beyond all human calculation. The result is the ocean currents. But how much is due to one cause and how much to another in the present state of knowledge no man can tell. Only for a few years have the phenomena of ocean currents been made the object of scientific observation and research. But the effect of ocean currents in modifying climate is well understood, and the modification of climate means nothing else than the transfer of heat. This is all that I have to say of the rivers of the sea, and if there are no more questions, we will now look at the movement of heat caused by icebergs.”

No question was asked, and Mr. Wilton continued:

“In polar regions there must be an immense formation of ice. Except in the oceans, the movements of water are chiefly movements of water in the condition of ice. Only for a small part of the year could water exist unfrozen. Immense regions of the Antarctic continent seem to be covered with one broad glacier. The ice pushes down into the sea until, undermined by the dashing of the waves, it breaks off, and enormous fragments are launched upon the deep waters. Sir James Ross saw in the southern ocean a chain of such icebergs extending as far as the eye could reach from the mast-head, many of them from one hundred feet to one hundred and eighty feet in height and miles across. Captain d’Urville saw one thirteen miles long and one hundred feet high. Its bulk was so vast that though the waves were dashing against it not a tremor was perceptible. Astronomic observations could be made from it as if it were solid rock rooted in the heart of the earth. In the same manner icebergs are formed in the northern ocean also. How much heat is given out in the freezing of water?”

“About one hundred and forty degrees,” answered Peter.