I dare say that many of those present will, in the course of their
lives, have opportunities of seeing some of the great meteors, or
fire-balls, which are occasionally displayed. Generally speaking, about
one hundred or so of these splendid objects are recorded every year.
We are never apprised that they are coming; they take us unawares, and
therefore we have no opportunity to make proper arrangements for seeing
them. There is only the chance that such persons as have been fortunate
enough to see them will have noted the circumstances with sufficient
accuracy to enable us to make use of their observations.
[Illustration: FIG. 78.--How to find the Height of a Meteor.]
The chief point to determine is the height of the meteor above the
earth. For this we must have two observations at least, made in places
as far asunder as possible. Suppose an observer at London and an
observer at York were both witnesses of a splendid meteor; if they
find, on subsequent comparison, that their observations were made at
the same moment, there is no reasonable doubt that it was the same
object they both saw. The observer at York describes the meteor as
lying to the south, halfway down from the point directly over his head
towards the horizon. The London observer speaks of the meteor as being
to the north; and to him also it appeared that the object was halfway
down towards his horizon from the point directly over his head. If you
know a little Euclid, you can easily show from these facts that the
height of the meteor must have been half the distance between London
and York, that is, 85 miles (Fig. 78).
I do not mean to say that the mode of discovering the meteor’s height
will be always quite such a simple process as it has been in the
case of the London and York observations. The principle is, however,
the same--that whenever from two sufficiently distant positions the
direction of the meteor has been observed, its path is known--just as
on p. 21 we showed how the height of the suspended ball was obtained
from observations at each end of the table. Generally speaking, bright
meteors begin at an elevation of between fifty and one hundred miles,
and they become extinguished before they are within twenty miles of the
ground.
Sometimes a tremendous explosion will take place during the passage of
a meteor through the air. There was a celebrated instance in America
on the 21st of December, 1876, which will give an idea of one of these
objects possessing exceptional magnificence. It began in Kansas about
seventy-five miles high, and thence it flew for a thousand miles at a
speed of ten or fifteen miles a second, until it disappeared somewhere
near Lake Ontario. Over a certain region between Chicago and St.
Louis, the great ball of fire burst into a number of pieces, and
formed a cluster of glowing stars that seemed to chase each other over
the sky. This cluster must have been about forty miles long and five
miles wide, and when the explosion occurred a most terrific noise was
produced, so loud that many thought it was an earthquake. A remarkable
circumstance illustrates the tremendous height at which this explosion
occurred. The meteor had burst into pieces, the display was all over,
and was beginning to be forgotten, and yet nothing had been _heard_. It
was not until a quarter of an hour after the explosion had been _seen_
that a fearful crash was heard at Bloomington. The explosion actually
occurred 180 miles from the spot, and as sound takes five seconds to
travel a mile, you can easily calculate that the noise required a
quarter of an hour for its journey. What a tremendous noise it must
have been!
Shooting stars are of every grade of brightness. Beginning with the
more gorgeous objects which have been compared with the moon or
even with the sun himself, we descend to others as bright as Venus
or as Jupiter; others are as bright as stars of various degrees of
brilliancy. Fainter shooting stars are much more numerous than the
conspicuous ones; in fact, there are multitudes of these objects so
extremely feeble that the unaided eye would not show them. They only
become perceptible in a telescope. It is not uncommon while watching
the heavens at night to notice a faint streak of light dashing across
the field of the instrument. This is a shooting star which is invisible
except through the telescope.