Just as Mr. Faulkner pronounced the last words, he started in his chair, and assumed a stiff, dignified position, in a great hurry. "Bless my soul!" cried he—with a comic look of astonishment and vexation—"while I have been telling you what is the real secret of my interest in the sketch you have so kindly given to me, I have altogether forgotten that I came here to sit for my portrait. For the last hour, or more, I must have been the worst model you ever had to paint from!"
"On the contrary, you have been the best," said I. "I have been painting from your expression; and, while telling your story, you have unconsciously shown me the natural expression I wanted."
WHAT THE SUNBEAM DOES.
Heat, or the caloric portion of the sunbeam, is the great cause of life and motion in this our world. As it were with a magical energy, it causes the winds to blow and the waters to flow, vivifies and animates all nature, and then bathes it in refreshing dew. The intensity of the heat which we receive depends on the distance of the earth from the sun, its great source, and still more on the relative position of the two orbs; since in winter we are nearer the sun than we are in summer, yet, in consequence of the position of the earth at that season, the sun's rays fall obliquely on its northern hemisphere, rendering it far colder than at any other period of the year.
A great portion of the heat-rays which are emitted by the sun are absorbed in their passage through the atmosphere which surrounds our globe. It is calculated that about one-third of the heat-rays which fall on it never reach the earth, which fact adds another to the many beneficent purposes fulfilled by our gaseous envelope, screening us from the otherwise scorching heat. It is curious to trace the varied fates of the calorific rays which strike on the surface of the earth. Some at once on falling are reflected, and, passing back through the atmosphere, are lost amid the immensity of space; others are absorbed or imbibed by different bodies, and, after a time, are radiated from them; but the greater part of the beams which reach the earth during the summer are absorbed by it, and conveyed downward to a considerable distance, by conduction from particle to particle. Heat also spreads laterally from the regions of the equator toward the poles, thereby moderating the intense cold of the arctic and antarctic circles, and in winter, when the forest-trees are covered with snow, their deeply-penetrating roots are warmed by the heat, which, as in a vast store-house, has been laid up in the earth, to preserve life during the dreary winter. The rays which fall on the tropical seas descend to the depth of about three hundred feet. The sun's attraction for the earth, being also stronger at that quarter of the world, the heated waters are drawn upward, the colder waters from the poles rush in, and thus a great heated current is produced, flowing from the equator northward and southward, which tends to equalize the temperature of the earth. The sailor also knows how to avail himself of this phenomenon. When out at sea, despite his most skillful steering, he is in constant danger of shipwreck, if he fails to estimate truly the force and direction of those currents which are dragging him insensibly out of the true course. His compass does not help him here, neither does any log yet known give a perfectly authentic result. But he knows that this great gulf-stream has a stated path and time, and, by testing from hour to hour the temperature of the water through which he is proceeding, he knows at what point he is meeting this current, and reckons accordingly.
We have already said that heat was the producer of the winds, which are so essential to the preservation of the purity of the atmosphere. In order to understand their action, we shall consider the stupendous phenomenon of the trade-winds, which is similar to that of the current we have described. The rays of the sun falling vertically on the regions between the tropics, the air there becomes much heated. It is the property of air to expand when heated, and, when expanded, it is necessarily lighter than the cooler air around it. Consequently it rises. As it rises, the cooler air at once takes its place. Rushing from the temperate and polar regions to supply the want, the warm air which has risen flows toward the poles, and descends there, loses its heat, and again travels to the tropics. Thus a grand circulation is continually maintained in the atmosphere. These aerial currents, being affected by the revolution of the earth, do not move due north and south, as they otherwise would. Hence, while they equalize the temperature of the atmosphere, they also preserve its purity; for the pure oxygen evolved by the luxuriant vegetation of the equatorial regions is wafted by the winds to support life in the teeming population of the temperate zones, while the air from the poles bears carbonic acid gas on its wings to furnish food for the rich and gorgeous plants of the tropics. Thus the splendid water-lily of the Amazon, the stately palm-tree of Africa, and the great banyan of India, depend for nourishment on the breath of men and animals in lands thousands of miles distant from them, and, in return, they supply their benefactors with vivifying oxygen.
Little less important, and still more beautiful, is the phenomenon of dew, which is produced by the power of radiating heat, possessed in different degrees by all bodies. The powers both of absorbing and of radiating heat, in great measure, depend on the color of bodies—the darker the color, the greater the power; so that each lovely flower bears within its petals a delicate thermometer, which determines the amount of heat each shall receive, and which is always the amount essential to their well-being. The queenly rose, the brilliant carnation, the fair lily, and the many-colored anemone, all basking in the same bright sunshine, enjoy different degrees of warmth, and when night descends, and the heat absorbed by day is radiated back, and bodies become cooler than the surrounding air, the vapor contained in the atmosphere is deposited in the form of dew. Those bodies which radiate most quickly receive the most copious supply of the refreshing fluid. This radiating power depends on the condition of the surface, as well as upon color, so that we may often see the grass garden bathed in dew, while the gravel walks which run through it are perfectly dry, and, again, the smooth, shining, juicy leaves of the laurel are quite dry, while the rose-tree beneath it is saturated with moisture.
The great effect produced on the vegetable kingdom by the heat-rays may be judged of from the fact, that almost all the plants which exhibit the remarkable phenomena of irritability, almost approaching to animal life, are confined to those regions where the heat is extreme. On the banks of the Indian rivers grows a plant in almost constant motion. In the hottest of the conservatories at Kew is a curious plant, whose leaflets rise by a succession of little starts. The same house contains Venus's fly-trap. Light seems to have no effect in quickening their movements; but the effect of increased heat is at once seen. They exhibit their remarkable powers most during the still hot nights of an Indian summer.
Heat is of essential importance in the production and ripening of fruit. Many trees will not bear fruit in our cold climate, which are most productive in the sunny south. Animal as well as vegetable life is in great measure dependent on heat. Look at the insect tribes. The greater number of them pass their winter in the pupa state. Hidden in some sheltered nook, or buried in the earth, they sleep on, until the warmth of returning spring awakens them to life and happiness; and if, by artificial means, the cold be prolonged, they still sleep on, whereas, if they he exposed to artificial heat, their change is hastened, and butterflies may be seen sporting about the flowers of a hothouse, when their less favored relatives are still wrapped in the deepest slumber. To judge of the influence of heat on the animal and vegetable economy, we need but contrast summer and winter—the one radiant and vocal with life and beauty, the other dark, dreary, and silent.