irst we inspected a
great globe [A], graduated with meridians and parallels; we estimated that
three men would hardly be able to embrace its girth.... A second
instrument was a great sphere [B], not less in diameter than that measure
of the outstretched arms which is commonly called a geometric pace. It had
a horizon and poles; instead of circles it was provided with certain
double hoops (_armillae_), the void space between the pair serving the
purpose of the circles of our spheres. All these were divided into 365
degrees and some odd minutes. There was no globe to represent the earth in
the centre, but there was a certain tube, bored like a gun-barrel, which
could readily be turned about and fixed to any azimuth or any altitude so
as to observe any particular star through the tube, just as we do with our
vane-sights;[8]--not at all a despicable device! The third machine was a
gnomon [C], the height of which was twice the diameter of the former
instrument, erected on a very large and long slab of marble, on the
northern side of the terrace. The stone slab had a channel cut round the
margin, to be filled with water in order to determine whether the slab was
level or not, and the style was set vertical as in hour-dials.[9] We may
suppose this gnomon to have been erected that by its aid the shadow at the
solstices and equinoxes might be precisely noted, for in that view both
the slab and the style were graduated. The fourth and last instrument, and
the largest of all, was one consisting as it were of three or four huge
astrolabes in juxtaposition [D]; each of them having a diameter of such a
geometrical pace as I have specified. The fiducial line, or _Alhidada_, as
it is called, was not lacking, nor yet the _Dioptra_.[10] Of these
astrolabes, one having a tilted position in the direction of the south,
represented the equator; a second, which stood crosswise on the first, in
a north and south plane, the Father took for a meridian; but it could be
turned round on its axis; a third stood in the meridian plane with its
axis perpendicular, and seemed to stand for a vertical circle; but this
also could be turned round so as to show any vertical whatever. Moreover
all these were graduated, and the degrees marked by prominent studs of
iron, so that in the night the graduation could be read by the touch
without a light. All this compound astrolabe instrument was erected on a
level marble platform with channels round it for levelling.