In the most populous portions of the United States there are two periods of maximum efficiency and two of minimum each year. Let us consider that wonderful region including southern New England, the Middle Atlantic States, the Ohio Valley, and westward to the Rocky Mountains. Again referring to the records of Huntington we find that human energy is greatest in October; the output of factory, mine, and counting room is greater per man than at any other time of the year and the product of mental effort is greater and of higher quality. Likewise disease is less and the death rate the least. From this time there is a loss in energy until January or February, when vitality and efficiency may have dropped twenty to thirty per cent. Then there is a gain until May or early June, when the conditions of health and efficiency are nearly equal to the most favorable time of the year in October. Again there is a loss until the middle of July, when a second minimum occurs; physical and mental energy are at a low ebb and the death rate is high. Diseases are not quite the same as in winter, as stomach troubles are more common than colds. The hotter the summer and the colder the winter the less favorable are the conditions of human existence. As there is a certain optimum beyond which diurnal and annual range of temperature cannot increase without a loss in energy, so there is a limitation in latitude beyond which the favorable climatic conditions decrease as one goes northward or southward. As an example, Canada and northern Maine have but one unfavorable period, which is the entire winter. The people of these regions are at their greatest potential July to September, after which they show a steady decline as the severity of the northern winter draws upon their vitality, until in January and February their minimum is below that of regions considerably farther south for the same period. From the most favorable climatic area in the middle latitudes—and the entire world possesses none more favorable or of greater extent than that possessed by the United States—the loss of health and strength due to the enervating effects of heat, high humidity, and insufficient temperature oscillations increases as one goes toward the equator. In Florida and the southern third of the Gulf States there is but one favorable period, the short winter. The enervating conditions still further are manifest as one proceeds farther southward.