nd the measures of
Tycho Brahe, as reduced to systematic laws by Kepler, and finally by the
great Newton, made it clear that the Copernican theory was _true_: but no
one had succeeded in proving its truth in this particular way. Samuel
Molyneux must have been a man of great courage to set himself to try to
crack this hard nut; and we can understand the attraction which his
enterprise must have had for Bradley, who had just lost the beloved
colleague of many courageous astronomical undertakings. His co-operation
seems to have been welcomed from the first; his help was invited and
freely given in setting up the instrument, and he fortunately had the
leisure to spend considerable time at Kew making the observations with
Molyneux, just as he had been wont to observe with his uncle.

I must now briefly explain what these observations were. There is a bright
star [gamma] Draconis, which passes almost directly overhead in the
latitude of London. Its position is slowly changing owing to the
precession of the equinoxes, but for two centuries it has been, and is
still, under constant observation by London astronomers owing to this
circumstance, that it passes directly overhead, and so its position is
practically undisturbed by the refraction of our atmosphere.

[Sidenote: The instrument.]

[Sidenote: Expected results.]

It was therefore thought at the time that, there being no disturbance from
refraction, the disturbance from precession being accurately known, and
there being nothing else to disturb the position but "parallax" (the
apparent shift due to the earth's motion which it was desirable to find),
this star ought to be a specially favourable object for the determination
of parallax. Indeed it had been announced many years before by Hooke that
its parallax had been found; but his observations were not altogether
satisfactory, and it was with a view of either confirming them or seeing
what was wrong with them that Molyneux and Bradley started their search.
They set up a much more delicate piece of apparatus than Hooke had
employed. It was a telescope 24 feet long pointed upwards to the star, and
firmly attached to a large stack of brick chimneys within the house. The
telescope was not absolutely fixed, for the lower end could be moved by a
screw so as to make it point accurately to the star, and a plumb-line
showed how far it was from the vertical when so pointing. Hence if the
star changed its position, however slightly,