If winds are due to such a simple mechanical causation as the production by the sun, of a rarefied atmosphere, the colder air rushing in from all sides into the empty spaces, we should hardly expect to find any definite currents bounded by well-defined limits; much less should we look for transverse and opposite currents going like messengers at varying rates of speed, some slow, and others exceedingly swift. Nor may stronger gales suddenly cease, as though stopped by some mighty invisible wall. And in no wise can they, from mere calorific agencies, leap out of perfect calmness into hurricane velocity, or subside into silence as by magic. On no such principle can they shift back upon their own track, going either way with terrific velocity. _A Great Cosmical System._ We have seen the marks of electrical action in the cases cited, and since we know something of the subtlety of the agent; that it may be "amassed, condensed and rarefied," that it is not loose and wandering, and the mere plaything of fortuitous forces, as the atmosphere is supposed to be; but, on the contrary, has close and most sympathetic adjustment with the earth-force; and that _it_ is the invisible hand that holds and manages the grosser atmospheric matter; since we know this, we are now brought to the study of a great cosmical system. FOOTNOTES: [13] Appendix, p. 105. CHAPTER X. SUN-SPOTS. _Grave Doubts._ HERBERT SPENCER says: "At present none of the interpretations of the sun-spots can be regarded as established." How numerous and how strange have been the theories promulgated as to the character of the manifestations called sun-spots. The dark spots in the sun have been supposed to be "solid bodies revolving very near its surface," "Smoke of volcanoes;" "Scum floating upon an ocean of fluid matter;" "Clouds;" "Opaque masses floating in the fluid matter of the sun, dipping down occasionally," "Fiery liquid surrounding the sun which, by its ebbing and flowing, the highest parts of it were occasionally uncovered, and appeared under the shape of dark spots, and by the return of the fiery liquid, they were again covered, and in a manner successively assumed different phases;" "Interruptions of continuity in the bright envelopes immediately surrounding the sun," "Cavities" etc. _Overestimate of the Degree of Spot-shadow._ Public sentiment in regard to the degree of darkness which is disclosed in sun-spots is exceedingly erroneous. It is believed that the spots are really dark. ZOeLLNER, however, states that "The black umbra of a spot emits four thousand times as much light as that derived from an equal area of the moon." "The blackest part of the spot is intrinsically bright." _What They are not, and what They are._