A Guide To The Scientific Knowledge Of Things Familiar Part 85

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 s.h.i.+P (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 s.h.i.+p above the line AC can be seen by the spectator A; the rest of the s.h.i.+p 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.