[Illustration: Here the width between the trees A and B will seem to be as great as the line AB: But the width between the trees C and D will seem to be no more than EF.] Q. _In a long straight STREET, WHY do the houses seem to APPROACH NEARER and nearer as they are more DISTANT?_ A. Because the more _distant the houses_ are, the more _acute will be the angle_ which any two make with our eye. Thus in the last figure-- If A and B were two houses at the top of the street, the street would seem to be as wide as the line A B: And if C and D were two houses at the bottom of the street, the street at the bottom would seem to be no wider than E F. Q. _In an AVENUE of TREES, WHY do they seem to be SMALLER as their distance increases?_ A. Because the _further the trees are off_, the more _acute will be the angle_ made by their perpendicular height with our eye. [Illustration: Here the first tree A B will appear the height of the line A B; but the last tree C D will appear only as high as the line E F.] Q. _In a long straight STREET, WHY do the houses seem to be SMALLER and smaller the FURTHER they are OFF?_ A. Because the _further any house is off_, the more _acute will be the angle_ made by its perpendicular height with our eye. Thus in the last figure-- If A B be a house at the top of the street, its perpendicular height will be that of the line A B. If C D be a house at the bottom of the street, its perpendicular height will appear to be that of E F. 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_. [Illustration: 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. [Illustration: 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?_