We know that the cyclone is due to the excess of heat at the earth’s equator. It is true that this excess of heat is always in operation, whereas cyclones are not perpetually raging in sub-tropical climates. Ordinarily, therefore, the excess of heat does not cause tornadoes. Certain aerial currents are generated whose uniform motion suffices, as a rule, to adjust the conditions which the excess of heat at the equator would otherwise tend to disturb. But when through any cause the uniform action of the aerial currents is either interfered with or is insufficient to maintain equilibrium, then cyclonic or whirling motions are generated in the disturbed atmosphere, and propagated over a wide area of the earth’s surface. Now we recognize the reason of the excess of heat at the earth’s equator in the fact that the sun shines more directly upon that part of the earth than on the zones which lie in higher latitudes. Can we find any reason for suspecting that the sun, which is not heated from without as the earth is, should exhibit a similar peculiarity? Sir John Herschel considers that we can. If the sun has an atmosphere extending to a considerable distance from his surface, then there can be little doubt that, owing to his rotation upon his axis, this atmosphere would assume the figure of an oblate spheroid, and would be deepest over the solar equator. Here, then, more of the sun’s heat would be retained than at the poles, where the atmosphere is shallowest. Thus, that excess of heat at the solar equator which is necessary to complete the analogy between the sun spots and terrestrial cyclones seems satisfactorily established. It must be remarked, however, that this reasoning, so far as the excess of heat at the sun’s equator is concerned, only removes the difficulty a step. If there were indeed an increased depth of atmosphere over the sun’s equator sufficing to retain the requisite excess of heat, then the amount of heat we receive from the sun’s equatorial regions ought to be appreciably less than the amount emitted from the remaining portions of the solar surface. This is not found to be the case, so that either there is no such excess of absorption, or else the solar equator gives out more heat, in other words, is essentially hotter, than the rest of the sun. But this is just the peculiarity of which we want the interpretation. It may be taken for granted, however, that there is an analogy between the sun spots and terrestrial cyclonic storms, though as yet we are not very well able to understand its nature. Then next we come to one of the most interesting discoveries ever made respecting the sun—the discovery that the spots increase and diminish in frequency in a periodic manner. We owe this discovery to the laborious and systematic observations made by Herr Schwabe of Dessau.