We come now to Brueckner periods and sunspot cycles. The Brueckner periods have a length of about thirty-three years. Their existence was suggested at least as long ago as the days of Sir Francis Bacon, whose statement about them is quoted on the flyleaf of this book. They have since been detected by a careful study of the records of the time of harvest, vintage, the opening of rivers to navigation, and the rise or fall of lakes like the Caspian Sea. In his book on _Klimaschwankungen seit 1700_, Brueckner has collected an uncommonly interesting assortment of facts as to the climate of Europe for more than two centuries. More recently, by a study of the rate of growth of trees, Douglass, in his book on _Climatic Cycles and Tree Growth_, has carried the subject still further. In general the nature of the 33-year periods seems to be identical with that of the 11- or 12- year sunspot cycle, on the one hand, and of historic pulsations on the other. For a century observers have noted that the variations in the weather which everyone notices from year to year seem to have some relation to sunspots. For generations, however, the relationship was discussed without leading to any definite conclusion. The trouble was that the same change was supposed to take place in all parts of the world. Hence, when every sort of change was found somewhere at any given sunspot stage, it seemed as though there could not be a relationship. Of late years, however, the matter has become fairly clear. The chief conclusions are, first, that when sunspots are numerous the average temperature of the earth's surface is lower than normal. This does not mean that all parts are cooler, for while certain large areas grow cool, others of less extent become warm at times of many sunspots. Second, at times of many sunspots storms are more abundant than usual, but are also confined somewhat closely to certain limited tracks so that elsewhere a diminution of storminess may be noted. This whole question is discussed so fully in _Earth and Sun_ that it need not detain us further in this preliminary view of the whole problem of climate. Suffice it to say that a study of the sunspot cycle leads to the conclusion that it furnishes a clue to many of the unsolved problems of the climate of the past, as well as a key to prediction of the future.