Some of these locally super-heated spots were found to vary from the diameter of a few inches to thousands of miles—one near the solar equator, and visible from Earth, being nearly two hundred thousand miles across. These and other large spots are mostly situated between solar latitudes 5° and 35° north and south of the equator, and are so extensive that certain physical causes have made them more or less constant. The continued welling upward of these portions of the solar sea and their cooler return have banked up the outer sides or borders of the spots, and deepened their interior space, after the manner of a volcano, and they are in a state of incessant eruption or boil. Many of the sun-spots, also forming deep depressions, cavities, or wells in the photosphere, and penetrating for a considerable distance towards the interior of the sun, are caused by vast descending and often cyclonic cones of super-heated vapour of inconceivably enormous energy. Passing over the tops of the apertures, these are drawn inside and, once entered, spin round the whole interior surface with terrible velocity, causing the boiling lava-like contents to be involved in intense revolution, the speed of which cools the far edges on the surface of the photosphere, causing definite lines or boundaries of demarcation which, owing to their reduced temperature, though still liquid, are considerably subdued in colour. To the eye these present a darkened hollow of terrible depth and fierceness, in and through which mighty currents flow unceasingly with lightning rapidity, and in many cases several of these cyclonic seas are connected by straits or channels. Seen from above, they show a dark core, or ‘nucleus,’ and surrounding this is the ‘umbra,’ which is not so dark as the core but is really the darker and cooler _sides_ of the cavity; and between this and the blinding white of the outer sea, or surface of the photosphere, is the ‘penumbra,’ which is the _margin_ of the cavity, appearing a greyish white in contrast to the gleaming white outer surface, and these three lines of demarcation are easily distinguishable from Earth.