ations could not be properly made by men standing on the smooth
slanting floor of the gallery, it would be desirable to have
cross-benches capable of being set at different heights along the
sloping gallery. In some observations, indeed, as where the transits of
several stars southing within short intervals of time had to be
observed, it would be necessary to set some observers at one part of the
gallery, others at another part, and perhaps even to have several sets
of observers along the gallery. And this suggests yet another
consideration. It might be thought desirable, if great importance was
attached (as the whole building shows that great importance must have
been attached) to the exactness of the observations, to have several
observations of each transit of a star across the mouth of the gallery.
In this case, it would be well to have the breadth of the gallery
different at different heights, though its walls must of necessity be
upright throughout--that is, the walls must be upright from the height
where one breadth commences, to the height where the next breadth
commences. With a gallery built in this fashion, it would be possible to
take several observations of the same transit, somewhat in the same way
that the modern observer watches the transit of a star across each of
five, seven, or nine parallel spider threads, in order to obtain a more
correct time for the passage of the star across the middle thread, than
if he noted this passage alone.

How far the grand gallery corresponds with these requirements can be
judged from the following description given by Professor Greaves in
1638:--"It is," he says, "a very stately piece of work, and not
inferior, either in respect of the curiosity of art, or richness of
materials, to the most sumptuous and magnificent buildings," and a
little further on he says, "this gallery, or corridor, or whatever else
I may call it, is built of white and polished marble (limestone), the
which is very evenly cut in spacious squares or tables. Of such
materials as is the pavement, such is the roof and such are the side
walls that flank it; the coagmentation or knitting of the joints is so
close, that they are scarce discernible to a curious eye; and that which
adds grace to the whole structure, though it makes the passage the more
slippery and difficult, is the acclivity or rising of the ascent. The
height of this gallery is 26 feet" (Professor Smyth's careful
measurements show the tru