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Hi Bill, hi John, hi diallists,
First of all, for those of you
who hadn't seen it yet I recommend Bill Thayer's page
on Vitruvius. It is carefully made and very well
translated. And second, thanks John
for your comments.
Now, the paragraph I referred to is as
follows:
8. Other kinds of winter-dials are made,
which are called Anaporica. They are constructed as
follows. With the aid of the analemma the hours are marked by brazen rods on
their face, beginning from the centre, whereon circles are drawn, shewing the
limits of the months. Behind these rods a wheel is placed, on which are measured
and painted the heavens and the zodiac with the figures of the twelve celestial
signs, by drawing lines from the centre, which mark the greater and smaller
spaces of each sign. On the back part of the middle of the wheel is fixed a
revolving axis, round which a pliable brass chain is coiled, at one of whose
ends a phellos or tympanum
hangs, which is raised by the water, and at the other end a counterpoise of sand
equal to the weight of the phellos.
9. Thus as the phellos ascends by the action of the water, the counterpoise of sand descends and turns the axis, as does that the wheel, whose rotation causes at times the greater part of the circle of the zodiac to be in motion, and at other times the smaller; thus adjusting the hours to the seasons. Moreover in the sign of each month are as many holes as there are days in it, and the index which in dials is generally a representation of the sun, shews the spaces of the hours; and whilst passing from one hole to another, it completes the period of the month. 10. Wherefore, as the sun passing through the signs, lengthens and shortens the days and hours, so the index of the dial, entering by the points opposite the centre round which the wheel turns, by its daily motions, sometimes in greater, at other times in less periods, will pass through the limits of the months and days. The management of the water, and its equable flow, is thus regulated. It is clear that Vitruvius is more interested
in reproducing the movement of the Sun by mechanical means than in telling the
time and that's why his dial is implemented in a different way than that
of Oughtred. But, in my opinion, there is the full idea of the Ougthred
dial: making a sundial using the ortographical projection (ie., that of an
astrolabe) instead of the classical gnomonical projection. What do you think of
it?
Happy Easter,
Anselmo Perez Serrada
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