A. Digging and ploughing help _to pulverize the soil_, by admitting _air into it_, and this increases its mean temperature. Q. _Why are FRANCE and GERMANY WARMER now, than when the vine would not ripen there?_ A. Chiefly because _their vast forests have been cut down_; and the soil is better _drained and cultivated_. Q. _What becomes of the WATER of PONDS and TUBS in summer-time?_ A. Ponds and tubs in summer-time are often left dry, because their water is _evaporated by the air_. Q. _How is this EVAPORATION PRODUCED and carried on?_ A. The air contains heat, and changes the _surface of the water into vapour_; this vapour (blending with the air) _is soon wafted away_; while _fresh_ portions of air _blow over the water_, and produce a _similar evaporation_; till the pond or tub is left quite dry. Q. _Why are the WHEELS of some machines kept CONSTANTLY WET with WATER_? A. _To carry off the heat_ (arising from _the rapid motion_ of the wheels) _by evaporation_, as soon as it is developed. Q. _Why is MOULD HARDENED by the SUN?_ A. Because (when the moisture of the mould has been _evaporated by the sun_) the earthy particles _come into closer contact_, and the mass becomes more solid. Q. _Show the WISDOM of GOD in this arrangement._ A. If the soil did not become _crusty and hard in dry weather_, the _heat and drought would penetrate the soil_, and kill both seeds and roots. Q. _Why is TEA cooled FASTER in a SAUCER than in a cup?_ A. Because _evaporation is increased_ by _increasing the surface_; and as tea in a saucer _presents a much larger surface to the air_, its heat is more rapidly carried off by evaporation. (The subject of "convection" will be treated of in a future chapter, and would scarcely be understood in this place.) Q. _Why is not the VAPOUR of the SEA SALT?_ A. Because the _salt_ is always _left behind_, by the process of evaporation. Q. _Why does a WHITE CRUST appear (in hot weather) upon CLOTHES wetted by sea water?_ A. The white crust is the _salt of the water_ left on the clothes by evaporation. Q. _Why does this WHITE CRUST always DISAPPEAR in WET weather?_ A. In _wet_ weather the _moisture of the air dissolves the salt_; and, therefore, it no longer remains visible. Q. _Why should NOT persons, who take violent exercise, WEAR very THICK CLOTHING?_ A. When the heat of the body is increased by exercise, _perspiration reduces the heat_ (by evaporation) _to a healthy standard_: as thick clothing _prevents this evaporation_, and confines the heat and perspiration _to the body_, it is injurious to health. CHAPTER XIII. COMMUNICATION OF HEAT. 1.--CONDUCTION. Q. _How is HEAT COMMUNICATED from one body to another?_ A. 1. By Conduction. 2. By Absorption. 3. By Reflection. 4. By Radiation: and 5. By Convection. Q. _What is meant by CONDUCTION of heat?_ A. Heat communicated from one body to another, _by actual contact_.