Excessive dryness of the skin, which is a common cause of discomfort, is not very closely related to the humidity of the air. “In winter,” says Hill, “if there be a wind the rate of evaporation is so accelerated that the skin feels dry, because in order to check the loss of body heat the sweat glands are inactive and the blood vessels of the chilled skin are constricted.” There has been a great deal of discussion about the dry air of American buildings in winter, and startling figures have been adduced to show that the air of such buildings is dryer than that of deserts. So far as measurements of relative humidity go, this is perfectly true; but, as Dr. G. T. Palmer, of the New York State Commission on Ventilation, has pointed out, there is an important difference between dryness and “dryingness.” The latter depends upon the movement of the air, as well as the relative humidity. The circulation of the air in a desert is generally much more active than that of the air in a building with the windows shut, and therefore much more conducive to rapid evaporation. There are systems of ventilation in which the air is kept in steady and rapid motion, and it is probably only in such cases that the air of our heated houses can be compared to that of a desert. From the European point of view American buildings are notoriously overheated, but this is probably due to the fact that our hot summers--much hotter than those of Europe--have adapted us to a tropical climate. It is natural to inquire whether the atmospheric conditions that affect the comfort of man do not also exercise a marked influence upon his muscular efficiency and his mental powers. This question has been answered in the affirmative by a number of ingenious writers, who have sought to establish definite quantitative relations between certain states of the atmosphere and the output of work in factories, the grades attained by school children, etc. Thus, Dr. Ellsworth Huntington, a well-known worker in this field, declares that the most favorable daily mean temperature for mental activity (the temperature being measured out-of-doors) is about 40 degrees F., and for physical activity about 60 degrees F. Contrary to the common opinion, he holds that our general efficiency is at low ebb in midwinter and fairly high in summer. Variability in temperature, within certain limits, he finds to be stimulating; equable temperature the reverse. He has drawn charts showing the distribution of what he calls “climatic energy”--i. e., the combination of certain weather factors supposed to control human efficiency--throughout the world, and other charts showing a more or less similar distribution of “civilization.” He has also made an ambitious attempt to interpret the history of mankind in terms of weather and climate.