A Guide To The Scientific Knowledge Of Things Familiar Part 25

A. If the balloon be 3 square feet in size, it is _lighter_ (when inflated) than 3 square feet of _common air_, and therefore _floats through it_; as a cork (at the bottom of a tub of water) would rise to the surface.

Q. _Why does SMOKE RUSH UP a CHIMNEY?_

A. The heat of the fire _expands the air in the chimney_; and (being thus made _lighter_ than the air around), it _rises up the chimney_, and carries the smoke in its current.

Q. _Why has a LONG CHIMNEY a greater DRAUGHT than a short one?_

A. Because air rises faster and faster the _higher it ascends_ in a chimney flue; the same as a stone falls faster and faster the _nearer it approaches to the ground_.

Q. _Why will a LONG chimney SMOKE, unless the FIRE be pretty FIERCE?_

A. If the fire be not pretty fierce, its heat will not be sufficient to _rarefy all the air in the chimney_; and then the chimney will _smoke_.

Q. _Why will the chimney smoke, if the fire be not BIG enough to heat ALL the air in the CHIMNEY FLUE?_

A. Because the _cold air_ (condensed in the upper part of the flue), _will sink from its own weight_, and sweep the ascending smoke _back with it_ into the room.

Q. _What is the use of a COWL upon a chimney-pot?_

A. The cowl acts as a _screen against the wind_, to prevent it from blowing into the chimney.

Q. _What HARM would the WIND do, if it were to BLOW into a CHIMNEY?_

A. 1st--It would prevent the smoke from getting out: and

2ndly--The _cold air_ (introduced into the chimney by the wind) _would fall down the flue_, and drive the smoke with it _back into the room_.

Q. _Why does a SMOKE-JACK turn round in a chimney?_

A. The current of hot air up the chimney, striking against the _oblique vanes of the smoke-jack_, drives them round and round; in the same way as the sails of a _wind-mill_ are driven round by the _wind_.

Q. _Why are some things SOLID, others LIQUID, and others GASEOUS?_

A. As _heat_ enters any substance, _it drives its particles further asunder_; and a _solid_ (like _ice_) becomes a _liquid_; and a _liquid_ (like _water_) becomes a _gas_.

Q. _Why does WATER SIMMER before it boils?_

A. The particles of water _near the bottom of the kettle_ (being formed into _steam_ sooner than the rest) _shoot upwards_; but are _condensed_ again (as they rise) _by the colder water_, and produce what is called "simmering."

Q. _What is meant by SIMMERING?_

A. A gentle tremor or _undulation_ on the surface of the water. When water _simmers_, the bubbles _collapse beneath the surface_, and the steam is condensed to _water again_: but when water _boils_, the bubbles _rise to the surface_, and _steam is thrown off_.

Q. _Why does a KETTLE SING when the water simmers?_

A. Because the _air_ (entangled in the water) escapes by _fits and starts_ through the _spout of the kettle_; which makes a noise like a wind instrument, when it is blown into.

Q. _Why does NOT a kettle SING, when the water BOILS?_

A. As _all_ the water is _boiling hot_, the steam meets with no _impediment_, but freely escapes in a continuous stream.

Q. _When does a kettle sing most?_

A. When it is set on a _hob_ to boil.

Q. _Why does a kettle SING MORE when it is set on the SIDE of a fire, than when it is set in the MIDST of the fire?_

A. When the kettle is set on the _hob_ to boil, the heat is applied very _partially: one side is hotter than the other_, and therefore the steam is more _entangled_.

Q. _Why does a KETTLE sing, when the boiling water begins to COOL again?_

A. Because the _upper_ surface cools _first_; and the steam (still rising from the lower parts of the kettle) is _again entangled_, and escapes fitfully.

Q. _Why does BOILING WATER SWELL?_

A. Water (like air) _expands by heat_. The heat of the fire drives the particles of water _further apart from each other_; and (as they are not _packed so closely together_) they take up _more room_; or (in other words) the water _swells_.

Q. _What is meant when it is said, "that HEAT drives the PARTICLES of water further APART from each other."_

A. Water is composed of little globules, like very small grains of sand; the heat _drives_ these particles _away from each other_; and (as they then require more _room_) the water _swells_.

Q. _Why does BOILING WATER BUBBLE?_

A. Water contains _air_; and (as the water is heated) _the air is driven out_, and raises a _bubble_ in that part of the water which resists its escape.

Q. _Why does a KETTLE sometimes BOIL OVER?_

A. Liquids _expand very much by heat_; if, therefore, a kettle be _filled with cold water_, some of it must _run over_ as soon as it is _expanded by heat_.