The intensity of sunlight at each point of the Sun's surface is variously expressed as 190,000 times that of a standard candle, 5300 times that of the metal in a Bessemer converter, 146 times that of a calcium light, or, 3·4 times that of an electric arc. The same authority estimates at 30 _calories_ the value of the _Solar Constant_; that is to say, the heat which, if our atmosphere were removed, would be received from the Sun in a minute of time upon a square metre of the Earth's surface that had the Sun in its zenith, would be sufficient to raise the temperature of a kilogram of water 30 degrees Centigrade. This would involve that the heat radiation from each square metre of the Sun's surface would equal 1,340,000 calories; or sufficient to melt through in each minute of time a shell of ice surrounding the Sun to the thickness of 58·2 feet. Prof. Abbot's most recent determination of the solar constant diminishes these estimates by one third; but he still gives the probable temperature of the solar surface as not far short of 7000 degrees Centigrade, or about 12,000 degrees Fahrenheit. The Sun, then, presents us with temperatures and pressures which entirely surpass our experience on the Earth. The temperatures, on the one hand, are sufficient to convert into a permanent gas every substance with which we are acquainted; the pressures, on the other hand, apart from the high temperatures, would probably solidify every element, and the Sun, as a whole, would present itself to us as a comparatively small solid globe, with a density like that of platinum. With both factors in operation, we have the result already given: a huge globe, more than one hundred times the diameter of the Earth, yet only one-fourth its density, and gaseous probably throughout the whole of its enormous bulk. What effect have these two factors, so stupendous in scale, upon its visible surface? What is the appearance of the Sun?