582. _The surplus heat should be removed equally from all parts of the system._ The rapid evaporation of fluids, as in free perspiration, or from radiation, as in a cold atmosphere, is attended with a removal of heat from the system. This modifies the action of the circulatory vessels. Consequently, if heat is suddenly and rapidly abstracted from one part of the system, the equilibrium of the circulation is destroyed, which will produce disease. -=-=-=-=-=-=-=-=-=-=-=-= Why do we, then, require more clothing in winter than in summer? 581. Why do persons of firm health and vigorous constitutions need less clothing than those who are feeble? What is a general practice among infirm persons? What would be more judicious? 582. Why should the surplus heat be removed equally from all parts of the system? What is said respecting currents of air from small apertures? -=-=-=-=-=-=-=-=-=-=-=-= _Observation._ Currents of air that impinge upon small portions of the body, as from small apertures, or from a window slightly raised, should be avoided. They are more dangerous than to expose the whole person to a brisk wind, because the current of air removes the heat from the part exposed, which disturbs the circulation of blood and causes disease, usually in the form of "colds." For the same reason, it is not judicious to stand in an open door, or the opening of a street. 583. _The system suffers less when the change of temperature is gradual._ The change in the production of heat, as well as in the evaporation of fluids from the system, is gradual when not influenced by foreign causes. This gradual change is known under the name _acclimation_. By this means the body is enabled to endure tropical heat and polar cold. Owing to this gradual adaptation of the system to different temperatures, we can bear a greater degree of heat in the summer between the tropics, than in the winter under the polar circles. On the other hand, we can endure a greater degree of cold in winter and in the arctic region, than in the summer and in equatorial countries. 584. The sensation of heat which would be oppressive in a mild, warm day of January, would only be grateful in July, and a degree of cold which could scarcely be endured in August, would not be uncomfortable in December. The changes of season in our latitude prevent the disagreeable and perhaps fatal consequence that would follow, if no spring or autumn intervened between the severity of winter's cold and the intensity of summer's heat. During the transition periods, the constitution is gradually changed, and adapted to bear the extremes of temperature without suffering. The amount of heat generated in the nutrient capillary vessels, is likewise diminished or increased as the temperature of the season becomes greater or less. -=-=-=-=-=-=-=-=-=-=-=-=