Q. _Why can a SWAN or DUCK dive under water WITHOUT being WETTED?_ A. Because their feathers are covered _with an oily secretion_, which repels the water. Q. _What is the cause of MIST?_ A. When currents of air _from land_ mix with currents of air _from water_, the currents _from the water are condensed into mist_ by the colder currents _blowing from the land_. Q. _Why are the currents of air from the LAND COLDER than those blowing over WATER?_ A. Because the earth _radiates heat very freely_, and (being greatly cooled down) _cools the air also_ which comes in contact with it. Q. _Why is not the AIR, which passes over WATER, so COOL as that which passes over LAND?_ A. Because _water does not cool down at sun-set_, so fast as the _land_ does; and, therefore, the air in contact with it is _warmer_. Q. _Why does not WATER cool down so fast as LAND?_ A. 1st--Because the _surface_ of water is _perpetually changing_, and as fast as _one_ surface is made cold, _another_ is presented: and 2ndly--The moment water is made cold _it sinks_, and _warmer portions of water rise to occupy its place_: therefore, before the _surface of water is cooled_, the _whole volume_ must be made cold; which is not the case with land. Q. _What is the cause of a "pea-soup" LONDON FOG?_ A. These fogs (which occur generally in the winter time) are occasioned thus:--Some current of air (being suddenly _cooled_) _descends into the warm streets_, preventing the rise of the smoke, and _forcing it back in a mass_ towards the earth. Q. _Why are there not ALWAYS FOGS every night?_ A. Because the air will always hold in solution a certain quantity of vapour, (which varies according to its temperature): and when the air is _not saturated with vapour_, it may be condensed without parting with it. Q. _Why are there EVER FOGS at night?_ A. If the air be _pretty well saturated with vapour_ during the day, as soon as its capacity for holding vapour _is lessened by the cold night_, it deposits some of the superabundant vapour in the form of dew or fog. Q. _Why is there very OFTEN a fog over MARSHES and RIVERS at night-time?_ A. The air of marshes is almost _always near saturation_; and, therefore, the _least depression of temperature_, will compel it to relinquish some part of its moisture in dew or fog. Q. _What is the DIFFERENCE between DEW and RAIN?_ A. In _dew_, the condensation is made _near the earth's surface_: In _rain_, the drops fall _from a considerable height_; but the cause of both is the same, viz.--COLD _condensing the vapour of the air_, when it is near the point of _saturation_. Q. _Why does MIST and FOG VANISH at sunrise?_ A. Because the condensed particles are again _changed into invisible vapour_, by the heat of the sun. Q. _What is the difference between a MIST and FOG?_ A. MIST is generally applied to _vapours condensed on marshes, rivers_, and _lakes_.