=Let Mother Earth Cool and Refresh You During the Heat of Summer.= How little the average man realizes the possibilities for improving his condition that lie close at hand. He does not know, or he is indifferent to the fact, that only three feet from the surface of the ground it is as cool at midday as at midnight, and that there is no diurnal variation in temperature below that depth, and no annual variation below a depth of from thirty to forty feet. If one were to set down the temperature of each day, add the numbers at the end of the year, and divide the sum by 365 the quotient would equal the temperature always found at that place at a depth of about thirty feet. The temperature of a deep-flowing spring is always about the mean annual air temperature of the place. Here is health-giving coolness for summer and warmth for winter of which one takes little heed and derives practically no profit. Remarkable, is it not? And these beneficent conditions are universal and available for all, except to those crowded into congested centers of population. The temperature is 54° in the Mammoth Cave in Kentucky and shows no change from day to day and from winter to summer. During the extreme heat of summer and the cold of winter many could profitably, healthfully, and pleasantly live below ground. During such periods the cellar of the house, which should be deep and spacious, even extending beyond the dimensions of the edifice above, if a continuous supply of pure air could be forced through it, or natural ventilation accomplished by the plan outlined below, should be the lounging, resting, and sleeping place of the occupants of the household. It is not impossible or extremely difficult to change the stagnant, moist, germ-laden, ill-smelling air of the average cellar, in which it is positively dangerous to spend much time, into active, pure, and delightfully healthful air,—air in which the worn and weary worker from the heat of the farmer’s field, or the artisan and the clerk from the debilitating temperatures of the factory and the office could recuperate from the toil of the day, and from which they would go forth each morning invigorated for another day’s efficient service, instead of dragging weary limbs from hot, sleepless beds, each morning less in energy than the day before. As is shown in other parts of this book, the researches of Huntington have proven conclusively that man is at his lowest physical and mental points of efficiency, and more subject to the contraction of disease through weakness, in midsummer and midwinter, and that the hotter the summer and the colder the winter the less is his energy and the lower is his power of resistance.