A Guide To The Scientific Knowledge Of Things Familiar Part 70

Q. _Why does WIND DRY damp LINEN?_

A. Because dry wind (like a dry sponge) imbibes the particles of vapour from the surface of the linen, as fast as they are formed.

Q. _Which is the HOTTEST PLACE in a church, chapel, or theatre?_

A. The gallery.

Q. _Why is the GALLERY of all public places HOTTER than the lower parts of the building?_

A. Because the heated air of the room _ascends_, and all the _cold air_ (which can enter through the doors and windows) _keeps to the floor_, till it has become heated.

Q. _Why do PLANTS often grow out of WALLS and TOWERS?_

A. Because sometimes the _wind_ blows the seed there with the dust; and sometimes _birds_ (flying over) drop the seed which they had formerly eaten.

CHAPTER XXIII.

BAROMETER.

Q. _What is a BAROMETER?_

A. A weather-gla.s.s, or instrument to show the changes of the weather, by marking the _variations in the weight of air_.

Q. _What is a THERMOMETER?_

A. An instrument to show how _hot or cold_ anything is.

Q. _What is the DIFFERENCE between a THERMOMETER and a BAROMETER?_

A. In a THERMOMETER the mercury is _sealed up from the air_:

In a BAROMETER the mercury is left _exposed_ (or open) to the air.

Q. _If the mercury of the thermometer be SEALED UP from the air, how can the air AFFECT it?_

A. The heat of the air pa.s.sing _through the gla.s.s tube_ into the mercury, causes it to _expand more or less_, and rise in the tube accordingly.

Q. _Why is the TUBE of a BAROMETER left OPEN?_

A. That the air may _press upon it_ freely; and as this pressure is _more_ or _less_, the mercury _rises or falls_ in the tube.

Q. _How can weather be affected by the WEIGHT of the air?_

A. When air is warm or moist, it is _lighter_ than usual:

When it is cold or dry, it is _heavier_: and as a barometer marks whether the air be _light_ or _heavy_, it indicates these _changes_.

Q. _How can you tell (by looking at a BAROMETER) what KIND of WEATHER it will be?_

A. Because the mercury in the tube _rises and falls_, as the air becomes lighter or heavier: and we can generally tell by the _weight_ of the air, what kind of weather to expect.

Q. _Does the WEIGHT of the air VARY MUCH?_

A. Yes; the atmosphere in England varies as much as _one-tenth part_ more or less.

Q. _What is the chief USE of a BAROMETER?_

A. To warn _sailors_ how to _regulate their s.h.i.+ps_, before squalls come on.

Q. _How can a BAROMETER warn SAILORS to regulate their s.h.i.+PS?_

A. As the barometer will tell when _wind_, _rain_, or _storm_ is at hand, the sailor can make his s.h.i.+p trim before it overtakes him.

Q. _Are there any RULES which can be depended on?_

A. Yes; there are _ten special rules_ to direct us how to know the changes of weather, by marking the mercury of a barometer.

Q. _What is the 1ST SPECIAL RULE in regard to the barometer?_

A. The barometer is _highest of all_ during a _long frost_; and it generally rises with a _north-east wind_.

Q. _Why is the barometer HIGHEST of all during a long FROST?_

A. Because long frost _condenses the air very greatly_; and the more air is _condensed_, the greater is its _pressure_ on the mercury of the barometer.

Q. _Why does the barometer generally RISE with a NORTH-EAST wind?_

A. Because NORTH-EAST winds make the air both _cold and dry_: the air, therefore, is both _condensed_, and _without vapour_.