A Guide To The Scientific Knowledge Of Things Familiar Part 36

Q. _Why does DRAINING land promote WARMTH?_

A. Because it _diminishes evaporation_; in consequence of which _less heat_ is abstracted from the earth.

Q. _Why does CULTIVATION increase the WARMTH of a country?_

A. 1st--Because _hedges and belts of trees_ are multiplied;

2ndly--Because the land is _better drained_;

3rdly--Because the land is _dug and ploughed;_ and

4thly--Because the vast _forests are cut down_.

Q. _Why do HEDGES and BELTS of TREES promote WARMTH?_

A. Because they _r.e.t.a.r.d evaporation_, by keeping off the _wind_.

Q. _If belts of trees promote WARMTH, why do FORESTS produce COLD?_

A. 1st--Because they _detain and condense the pa.s.sing clouds_:

2ndly--They prevent the access of both _wind and sun_:

3rdly--The soil of forests is always _covered with long damp gra.s.s, rotting leaves, and thick brushwood_: and

4thly--There are always many hollows in every forest _full of stagnant water_.

Q. _Why do LONG GRa.s.s and ROTTING LEAVES promote COLD?_

A. Because _they are always damp_; and the evaporation which they promote, _is constantly absorbing heat_ from the earth beneath.

Q. _Why do DIGGING and PLOUGHING help to make a country WARM?_

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 ma.s.s becomes more solid.

Q. _Show the WISDOM of G.o.d 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.