London:--"And
this is what I am now desirous to lay before this illustrious Society,
which I foretell will continue for ages, that I may explain beforehand
to young astronomers, who may, perhaps, live to observe these things, a
method by which the immense distance of the sun may be truly
obtained.... I recommend it, therefore, again and again to those curious
astronomers who, when I am dead, will have an opportunity of observing
these things, that they would remember this my admonition, and
diligently apply themselves with all their might in making the
observations, and I earnestly wish them all imaginable success--in the
first place, that they may not by the unseasonable obscurity of a cloudy
sky be deprived of this most desirable sight, and then that, having
ascertained with more exactness the magnitudes of the planetary orbits,
it may redound to their immortal fame and glory." Halley lived to a good
old age, but he died nineteen years before the transit occurred.

The student of astronomy who desires to learn how the transit of Venus
will tell the distance from the sun must prepare to encounter a
geometrical problem of no little complexity. We cannot give to the
subject the detail that would be requisite for a full explanation. All
we can attempt is to render a general account of the method, sufficient
to enable the reader to see that the transit of Venus really does
contain all the elements necessary for the solution of the problem.

We must first explain clearly the conception which is known to
astronomers by the name of _parallax_; for it is by parallax that the
distance of the sun, or, indeed, the distance of any other celestial
body, must be determined. Let us take a simple illustration. Stand near
a window whence you can look at buildings, or the trees, the clouds, or
any distant objects. Place on the glass a thin strip of paper vertically
in the middle of one of the panes. Close the right eye, and note with
the left eye the position of the strip of paper relatively to the
objects in the background. Then, while still remaining in the same
position, close the left eye and again observe the position of the
strip of paper with the right eye. You will find that the position of
the paper on the background has changed. As I sit in my study and look
out of the window I see a strip of paper, with my right eye, in front of
a certain bough on a tree a couple of hundred yards away; with my left
eye the paper is no longe