A Guide To The Scientific Knowledge Of Things Familiar - novelonlinefull.com
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Q. _Why does a man on the TOP of a MOUNTAIN or church spire seem to be no BIGGER than a CROW?_
A. Because the angle made by the _perpendicular height of the man_ (at that distance) _with our eye_, is no bigger than the perpendicular height of a _crow close by_.
[Ill.u.s.tration: Let AB be a man on a distant mountain or spire, and CD a crow close by:
The man will appear only as high as the line CD, which is the height of the crow.]
Q. _Why does the MOON appear to us so much BIGGER than the STARS, though in fact it is a great deal SMALLER?_
A. Because the moon is _very much nearer to us_ than any of the stars.
[Ill.u.s.tration: Let AB represent a fixed star, and CD the moon.
AB, though much the larger body, will appear no bigger than EF; whereas the moon (CD) will appear as big as the line CD to the spectator G.
The moon is 240,000 miles from the earth, not quite a quarter of a _million_ of miles. The nearest fixed stars are 20,000,000,000,000.
(i. e. 20 billions.)
If a ball went 500 miles an hour, it would reach the moon in twenty days: but it would not reach the nearest fixed star in 4,500,000 years.
Had it begun, therefore, when Adam was created, it would be no further on its journey than a coach (which has to go from the bottom of Cornwall to the top of Scotland) after it has past about three-quarters of a mile.]
Q. _Why does the MOON (which is a sphere) APPEAR to be a FLAT surface?_
A. It is _so far off_, that we cannot distinguish any difference between the _length of the rays_ which issue from the _edge_, and those which issue from the _centre_.
[Ill.u.s.tration: The rays AD and CD appear to be no longer than the ray BD; but if all the rays seem of the same length, the part B will not seem to be nearer to us than A and C, and therefore ABC will look like a flat or straight line.
The rays AD and CD are 240,000 miles long.
The ray BD is 238,910 miles long.]
Q. _Why do the SUN and STARS (which are spheres) appear to be FLAT surfaces?_
A. Because they are such an _immense way off_, that we can discern _no difference of length_ between the rays which issue from the _edge_, and those which issue from the _centre_ of these bodies.
The rays AD and CD appear no longer than BD; and as B appears to be no nearer than A or C, therefore ABC must all seem equally distant; and ABC will seem a flat or straight line. (See last figure.)
Q. _Why does DISTANCE make an object INVISIBLE?_
A. Because the angle (made by the _perpendicular height_ of the distant object _with our eye_) is so very _acute_, that _one_ line of the angle _merges in the other_.
[Ill.u.s.tration: Here the tree AD would not be visible to the spectator C, even if he were to approach as far as B; because no visible perpendicular can be inserted between the two lines AC, DC, till after the point B is past; when the tree will appear like a very little speck.]
Q. _Why do TELESCOPES enable us to SEE objects INVISIBLE to the naked eye?_
A. Because they _concentrate several rays_ within the tube of the telescope, and _bend them upon the mirror or lens_, which acts as a _magnifying gla.s.s_.
Q. _When a SHIP (out at sea) is approaching the sh.o.r.e, why do we SEE the small MASTS before we see the bulky HULL?_
A. Because the _earth is round_, and the _curve_ of the sea _hides the hull_ from our eyes, after the tall _masts_ have become visible.
[Ill.u.s.tration: Here only that part of the ship above the line AC can be seen by the spectator A; the rest of the ship is hidden by the swell of the curve DE.]
Q. _What is meant by REFRACTION?_
A. The _bending of a ray of light_, as it pa.s.ses from one medium to another.
Q. _How is a ray of light BENT, as it pa.s.ses from one medium to another?_
A. When a ray of light pa.s.ses into a _denser_ medium, it is bent _towards the perpendicular_. When it pa.s.ses into a _rarer_ medium, it is bent _from_ the perpendicular.
[Ill.u.s.tration: Suppose DE to be a perpendicular line.
If AB (a ray of light,) enters the water, it will be bent _towards_ the perpendicular to C.
If (on the other hand) CB (a ray of light) emerges _from_ the water, it would be bent _away from_ the perpendicular towards A.]
Q. _Why does a SPOON (in a gla.s.s of water) always appear BENT?_
A. Because as the light of the spoon _emerges from the water_, it is _refracted_.
And the spoon looks like ABC. (See the last figure.)
Q. _Why does a river always appear more shallow than it really is?_
A. Because the light of the bottom of the river is REFRACTED as it emerges out of the water: and (as a stick is not so long when it is _bent_, as it is when it is _straight_) so the river seems less deep than it really is.
Q. _How much deeper is a river than it seems to be?_
A. One-third. If, therefore, a river seems only 4 feet deep, it is really 6 feet deep.
N. B. Many boys get out of their depth in bathing, in consequence of this deception. Remember, a river is always one-third deeper than it appears to be:--thus, if a river seems to be 4 feet deep, it is in reality 6 feet deep, and so on.
