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ance not unlike that of an orange skin. But, under favorable circumstances and with a telescope of high power, the solar disk is found to be covered with small, intensely bright bodies irregularly distributed.

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These are now known as rice-grains.*

They are

often apparently crowded together in luminous ridges, or streaks, termed faculæ (facula, a torch); while the rice-grains themselves, according to Prof. Langley, are composed of granules.

Minute as a

Various observers describe the solar surface differently. A peculiar, elongated, leaf-shaped appearance of the rice-grains, called the willow-leaf structure, is shown in Fig. 17, as seen by Nasmyth. Newcomb compares the sun's appearance to that of a plate of rice-soup. Young says it frequently resembles bits of straw lying parallel to one another-the "thatched-straw formation."

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Typical Sun-spot, of Dec. 1873, showing the filaments pointing to the center.

granule seems, probably the smallest has a diameter of, at least, 100 miles.

Physical Constitution of the Sun.*-Of the constitution of the sun, and the cause of the solar spots, very little is definitely known.

WILSON'S THEORY supposed that the sun is composed of a solid, dark globe, surrounded by three atmospheres. The first, nearest the black body of the sun, is a dense, cloudy covering, possessing high reflecting power. The second is called the photosphere. It consists of an incandescent gas, and is the seat of the light and heat of the sun, being the sun that we see. The third, or outer one, is transparent-very like our atmosphere.

According to this theory, the spots are to be explained in the following manner. They are simply openings in these atmospheres made by powerful upward currents. At the bottom of these chasms, we see the dark sun as a nucleus at the center, and around this the cloudy atmosphere-the penumbra. This explains a black spot with its penumbra. Sometimes the opening in the photosphere may be smaller than that in the inner or cloudy atmosphere; in that case there will be a black spot without a penumbra.

It will be natural to suppose that when the heated gas of the photosphere, or second atmosphere, is violently rent asunder by an eruption or current from below, luminous ridges will be formed by the heaped-up gas on every side of the opening. This would account for the faculæ surrounding the sun

*For the views of various authorities on the constitution of the sun, solar spots, etc., see Newcomb's Astronomy, third edition, p. 271.

spots. It will be natural, also, to suppose that sometimes the cloudy atmosphere below will close up first over the dark surface of the sun, leaving only an opening through the photosphere, disclosing at the bottom a grayish surface of penumbra. We can

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readily see, also, how, as the sun revolving on its axis brings a spot nearer and nearer to the center, thus giving us a more direct view of the opening, we can see more and more of the dark body. Then as it passes by the center the nucleus will disappear,

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