A sun-spot, we thus perceive, is an elongated wave or ridge of eruption along the rotational direction of the sun's body. Why, then, it may be asked, is not this line of eruption continuous entirely around the sun? For the same reason, it may be answered, that our own cyclones are not continuous, though caused substantially in the same manner, and that volcanic eruptions only occur at long intervals, though the forces at work are continuous. Lowering of temperature follows swiftly after eruption, and as the deeper structures of the solar nucleus become gradually affected, instead of volatilization of the outer layers of the surface, we will have diffused gaseous expansion of large portions, and finally of the entire solar mass, which cannot as a whole be volatilized by any conceivable planetary energy. We see these operations exemplified in heating a bar of copper in a Bunsen flame; the latter first turns green from surface volatilization of the copper, but as the heat is communicated to the deeper structures the green flame disappears, and the whole additional heat goes to raise the temperature of the mass. These processes in the sun are thus seen to be self-compensatory in their nature. They are the means provided to distribute the restricted areas of abnormally heated photosphere over the solar surface, and finally to cause the absorption of the whole excess of heat in the sun's central mass. The balance is so evenly maintained, however, that, were all the planets equally distributed with reference to the sun's surface, such sun-spots would be the exception and not the rule, and their distribution would be equal and constant; but, as the planets continually change their positions with reference to the sun and to each other, only by some such provision of nature could the internal structure of the sun be maintained without serious derangement, or, indeed, final disruption. So nature distributes her stores of heat upon the earth. These beautiful self-compensations we shall find suddenly appearing, as we advance, in all parts of the field of astronomical research.