Since the extensive system of observations by the Weather Bureau was inaugurated, some fifty years ago, it has been revealed to us that frequently the Ohio Valley would suffer a deficit in rainfall that would persist for periods as great as five or six years, while New England and the South Atlantic States, or other large areas of the country, had an excess. This is an illustration that excesses in one part of the country were balanced by shortages in other parts that occurred at the same time. But the long-period oscillations in climate that are measured in hundreds of years instead of tens—these changes seemed to have occurred simultaneously in the middle latitudes of Europe and America. These changes were simultaneous in an east and west direction. Now we have evidence of such long-period changes in a north and south direction which were simultaneous, but of an _opposite character_, indicating that during the Christian Era the eastward track of storms has oscillated northward and southward. This would account for the occurrence of dry and of wet periods simultaneously throughout the vast stretch of territory between southern California and the Caspian Sea. In Guatemala, Yucatan, and other Central American countries there are ruins of cities and the evidence of an agriculture and a civilization that could not have been established with the torrential rains and jungle growths that now prevail in those regions. During the centuries when the big trees of California were receiving a large rainfall and making a thick annual growth, especially about the beginning of the Christian Era, because of a northward shifting of the climatic zone, the precipitation in Yucatan and Guatemala had so diminished as to leave only the amount of rainfall that could be economically employed in agriculture and in the rearing of great cities; and then, with a southward migration of the rain belt, these cities were suffocated with excessive precipitation, agriculture rendered impossible, and their temples and palaces buried beneath the gloom of a tropical growth.