Thus over the greater part of the temperate regions of the northern hemisphere we have evidence of an important rainy period between the extreme limits of 1800 B.C. and A.D. 400 or 500. This period was best developed from 1200 B.C. to A.D. 200, and reached its maximum about 400 B.C. It constitutes a remarkable wave of climatic variation, which is hitherto without adequate explanation. A somewhat similar, though less intense, wave which occurred about A.D. 1200-1300, and which is described in the following chapter, was associated by Wolf to a great outburst of sunspots which took place about A.D. 1200. It is well known that sunspots are an index of solar activity, the sun’s radiation being greater at times of spot maximum than at times of spot minimum. Greater solar radiation increases the evaporation over the oceans, so that the air becomes more humid. This moist air is carried by atmospheric currents over the land, where the moisture is condensed into clouds and greatly increases the rainfall. At the same time the cloud canopy shuts off some of the direct heat of the sun, and we have the curious paradox that at times of sunspot maximum, or greatest solar radiation, the temperature of the earth’s surface is lowest. The connexion outlined above is, however, extremely problematical for temperate regions. Since the absolute sunspot maximum at A.D. 1200 is also very doubtful, it will be realized that the evidence for the sunspot hypothesis of the mediæval rainfall maximum is extremely slender. Furthermore, since we know nothing whatever about the solar activity during the classical rainfall maximum, we are still less in a position to extend the sunspot hypothesis to that period also. The interesting theory recently put forward by O. Pettersson, already alluded to, provides a plausible alternative explanation of the severe stormy climate of the Peat-bog period, which reached a maximum near 400 B.C. Without going into details this theory is that the strength of the tides depends on the relative positions of the sun and moon, and the tides are greatest when these act in conjunction, and also when they are nearest to the earth. This fluctuation of strength passes through various cyclic variations with periods of nine years, about ninety years and about 1800 years, though the lengths of the periods are not constant. The latter cycle is most important to our purposes; according to Pettersson’s calculations the fluctuations of the “tide-generating force” were as follow: Maxima 3500 B.C. 2100 B.C. 350 B.C. A.D. 1434 Minima 2800 B.C. 1200 B.C. A.D. 530.