years between each transit, correspond
alternately to the month of June and the month of
December. The couples of transits are separated
from each other by an interval of time which
is alternately one hundred and five and one
hundred and twenty-two years. They all take
place shortly before one of the solstices—the
winter or the summer solstice—a circumstance
favourable for obtaining, by a wise selection of
points of observation, very considerable
differences in the duration of the phenomenon, as
seen from those diverse distant localities.

The last observation (by English astrono-
mers) is recorded in a prose idyll, to read
which takes you back thousands of years in
respect of facts, if not of time. A
retrogression of a thousand years would hardly
bring you to such a state of society as was
found, then alive and in the flesh, in the
enchanted isle of Otaheite. It was like finding
some region where fossil plants still grow, and
extinct animals still roam at liberty. Dear old
Captain Cook, we retain your spelling as
affectionately as we cherish your narratives. And
unfortunately there are no more such islands to
be discovered, nor ever will be—no more such
romantic voyages to be written. No more
sailors, landing at Botany Bay, will rush on
board in a fright at having seen the devil (a
kangaroo) with a body as big round as a barrel;
no more savages will be found polite to sailors,
believing them the representatives of the fair
sex of England.

Cook might well call the hill where the
observatory was fixed for watching the transit,
"Venus Point." Those, indeed, were days of
the Golden Age, inasmuch as his object in carrying
out astronomers to Otaheite was, that, by
observing the transit of Venus there, they might
determine the sun's parallax with greater
accuracy than heretofore.

The Sun plays so all-important a part in our
existence, that the interest attached to the
knowledge of his distance from the Earth is much
greater than would appear at first sight, and
considering it as a simple isolated fact. For
that distance serves to estimate the distances
of the heavenly bodies one from another.
Consequently, at every epoch, astronomers have
done their utmost to accomplish the measurement
of this fundamental distance.

In order to find the length of any unknown
distance, we must take some other length or
distance which we do know, and find out how
many times it is contained in the other. The
known distance which we use as our measure,
and which is called the base or the unity of our
measurement, is divided, if required, into a
certain number of equal parts, in case the
distance to be measured should not contain it an
exact number of times, and there should be a
remainder, which, of course, would be less than
the base or unity. Thus, to ascertain the length
of a wall, or a piece of stuff, you apply a yard
measure to it as many times as it will go; and then
you measure the remainder, if there be any, by
subdividing the yard into feet and inches. But
if we confined ourselves to the yard, or to any
other single unity, for the measurement of all
lengths, we should find much embarrassment in
applying it either to enormous or to minute
distances. With what precision can we figure
to ourselves a billion of yards, or of the millionth
part of a yard? We hear them named without
their impressing us with any definite idea.

In order to avoid excessively large numbers
leaving excessively small ones out of the
question, as they do not concern us on the present
occasion—we are obliged to replace the yard by
a larger unity, when considerable distances have
to be measured. Thus, roads are measured by
the mile. But if the yard is inapplicable to the
measurement of terrestrial distances, it is still
more useless for such distances as from star
to star. It is impossible to form any idea either
of those distances or of their relative proportions amongst themselves, unless we start from
some typical distance belonging to the same
order of magnitude as themselves.

For ascertaining the dimensions of surrounding
objects and their relative distances from
each other, a very natural proceeding is to take,
as a term of comparison and a unity of measurement,
some one part of the human body. Such
evidently was the origin of the unities of
measurement known as "cubits," "feet," "palms,"
&c. For journeys by sea and land, recourse
was had to unities of measure derived from the
dimensions of the terrestrial globe; such as the
ordinary league (the twenty-fifth part of a de-
gree, which is the three hundred and sixtieth
part of the Earth's circumference), and the
marine league (the twentieth part of a degree).
In these cases, the terrestrial globe is
substituted for the human body, to serve as a term
of comparison between the different distances
travelled on its surface.

But if, from these terrestrial distances, we
proceed to those which separate the stars, even
those which are nearest to us (always excepting
the Moon), the dimensions of our globe then
become much too small to serve as the unity of
measure for those enormous intervals of space.
We can only form a clear notion of their relative
lengths by comparing them with a unity of their
own class. The distance which separates us
from the Sun (for us the most influential of all
the heavenly bodies) becomes, then, naturally
the new term of comparison, the new unity of
measurement which we are induced to adopt.
The Sun's distance is determined by his parallax;
and his parallax is expected to be still
more accurately ascertained by observations of
the promised transit of Venus.

Parallax is the angle formed by an object
with two different observers placed at different
stations. Thus, suppose this letter A to be
greatly magnified, or to be traced on the surface
of a ten-acre field; fix an object, as a flag-staff,
at the apex, or top of the A, and an observer at
each of its feet, the angle formed by the legs of
the A will be, to them, the parallax of the flag-
staff. It will hence be clear that the nearer an
object is, the greater will be its parallax, the