A. Because the heat of the body is continually absorbed in converting the damp of the sheets into vapour; and as heat is abstracted from the body, its temperature is reduced below the healthy standard.
Q. Why do we not feel the same sensation of cold, if we throw a macintosh over our wet clothes?
A. The macintosh prevents evaporation, because the steam cannot escape through the air-tight fabric; and (as the wet cannot evaporate from the clothes) no heat is absorbed from our bodies.
Q. Why do not sailors get cold, who are so often wet all day with sea-water?
A. The salt of the sea retards evaporation; and (as the heat of the body is drawn off very gradually) the sensation of cold is prevented.
Q. Why does sprinkling a hot room with water cool it?
A. The heat of the room causes a rapid evaporation of the sprinkled water; and as the water evaporates, it absorbs heat from the room, and cools it.
Q. Why does watering the streets and roads cool them?
A. The hot streets and roads part with their heat to promote the evaporation of the water sprinkled on them.