Curiosities of Heat

by Lyman Beecher Tefft

“I think, sir, that the sun is the chief source of heat.”

“We certainly receive the larger part of our heat from the sun. No one can doubt this. So much of our heat comes from the sun that the temperature of the earth varies according to the sun’s heat, as if that were the only supply. If but a fleecy cloud pass between the sun and the earth, we feel a decided change of temperature. A few hours less of sunshine each day, and a few degrees more of inclination to the sun’s rays, change summer to winter and make the difference between the torrid and the frigid zones. Withdraw the heat of the sun altogether, and the whole world would become a desert of frozen death.”

“What is the cause of the sun’s heat?” asked Peter.

“You have asked a question which I cannot answer, and which no man can answer. The most careful and patient observations have been made to discover if possible the constitution of the sun; learned and curious conjectures have been brought forward to explain the source of its heat; but the positive results have not been very large. It is certain that the sun is a globe revolving upon its axis in a period of twenty-five days, nine hours, and thirty-six minutes. This is known by the motion of dark spots upon its surface. The appearance of the sun as seen through a telescope is that of a globe of fire, its surface often in a state of violent agitation and flecked here and there with dark, irregular, changeable spots. These spots are sometimes of enormous dimensions—thirty thousand or fifty thousand miles in diameter. They present a dark centre with a narrow border or penumbra of lighter shade. To account for these spots, it has been conjectured that the body of the sun is dark, but surrounded by a double envelope of clouds, the outer layer of which is intensely luminous. Openings in such enveloping clouds would present an appearance like the spots upon the sun. According to this supposition, the heat and light of the sun proceed, not from the body of the sun, but from this luminous enveloping cloud. But granting that this supposition is true, it gives no explanation of the origin of the sun’s heat. Laplace conjectured that the sun is a globe of fire in a state of violent, explosive conflagration, and that the spots are enormous crater-like caverns in its surface. Newton conjectured that comets falling into the sun and being consumed feed the solar fires and maintain its temperature. The reception of the dynamic theory of heat has led to the revival, in a modified form, of this conjecture of Newton. It is suggested that meteors or meteoric matter falling into the sun generates its heat by the force of concussion. To show that the intense heat of the sun might be thus generated, elaborate calculations have been made. It has been demonstrated that if the sun were a solid mass of anthracite coal, its combustion would maintain its heat at its present rate of emission only five thousand years, while the falling of the planet Jupiter into the sun would generate an equal amount of heat for thirty-five thousand years. A lump of coal falling from the earth to the sun would produce three thousand times more heat by the concussion than by its combustion.

“The nearest approach that has been made, of an exact and scientific kind, toward determining the constitution of the sun’s surface has resulted from an examination of the solar spectrum. A ray of light, by passing through a triangular prism of glass, is, as you know, divided into its elements, or constituent colors. The ray of light is spread out like a half-open fan. This divided and expanded ray, thrown upon a screen, is called the spectrum. An examination of the solar spectrum by a microscope shows certain fine dark lines across it. The lines are invariably the same in their position and grouping. The spectrum of the stellar light is found to differ from that of the solar light, and the light of one star differs from that of another star. Light from incandescent metallic vapors gives bright lines across the spectrum. Each metal has its own number, position, grouping, and color of these spectral lines. By comparing the solar spectrum with the spectra of the various metals—the processes are curious and the explanation difficult to be understood—corresponding lines are discovered, and the conclusion is reached that the sun’s atmosphere contains the vapors of several of our well-known metals, as iron, nickel, sodium, potassium, and others. This is a most curious and marvelous scientific feat, to make an approximate chemical analysis of the sun and stars by means of their light. The conclusions, however, seem trustworthy.

“Can you tell us, Ansel, whether the earth receives heat from the moon and stars?”

“I cannot, sir.”

“I should be glad, Mr. Hume, to have you instruct us upon this point.”

“In regard to the fixed stars,” answered Mr. Hume, “counting them as the remote suns of other planetary systems, we must believe that they radiate more or less heat upon the earth; some indeed have extravagantly maintained that we receive from them nearly as much heat as from the sun. The heat received from them is so small that we perceive no difference whether they be hidden, or shine with their utmost brilliancy. I do not know that investigations have been made to determine scientifically their exact thermal influence upon the earth. But little more can be said about the heat of the moon. The light of the full moon, concentrated by a two-foot burning-glass and thrown upon the bulb of the most delicate thermometer, produces no perceptible effect. By means of the electroscope or galvanometer, it is said, however, that the moon’s heat has been detected. At a late scientific convention held in Chicago, Prof. Elias Loomis read a paper, in which he stated that Mr. Harrison of England, by a comparison of observations made for sixteen years at Greenwich, nine years at Oxford, and sixteen years at Berlin, has discovered that the moon exerts a sensible influence upon the temperature of the earth, the highest temperature occurring from six to nine days after the new moon and the lowest about four days after the full moon. The conclusion, the opposite of what we should naturally expect—the higher temperature occurring when the enlightened face of the moon is turned from the earth—was explained by supposing the moon’s heat to be dark heat which would be absorbed by the vapors and the clouds, and thus tend to warm and dissipate them. By the dispersion of the clouds, the radiation of heat from the earth’s surface would go on more rapidly and the temperature would fall. According to this explanation, the lunar heat reduces instead of raising the temperature of the earth. The difference of temperature due to the moon’s influence Mr. Harrison believed to be two and a half degrees. Upon extending his calculations through forty-three years of observations made at Greenwich, he found the difference reduced to about one degree. As for myself, I confess myself still a skeptic touching the supposed influence of the moon upon temperature.”

“Upon that subject, I think,” said Mr. Wilton, “that we must wait patiently for more light. The popular superstitions which refer sickness and health, and every kind of good or evil fortune, to the benign or malignant influence of the moon, we, of course, must reject. Samuel, will you name the second chief source of heat?”