the center of gravity of Islamic astronomy shifted from the East to the
Hispano-Moorish West. At the same time there comes more evidence that
the line of mathematical protoclocks had not been left unattended. This
is suggested by a description given by Trithemius of another royal gift
from East to West which seems to have been different from the automata
and hydraulic devices of the tradition from Procopius to
Riḍwān:[19]
In the same year [1232] the Saladin of Egypt sent by his
ambassadors as a gift to the emperor Frederic a valuable
machine of wonderful construction worth more than five
thousand ducats. For it appeared to resemble internally a
celestial globe in which figures of the sun, moon, and
other planets formed with the greatest skill moved, being
impelled by weights and wheels, so that performing their
course in certain and fixed intervals they pointed out the
hour night and day with infallible certainty; also the
twelve signs of the zodiac with certain appropriate
characters, moved with the firmament, contained within
themselves the course of the planets.
[Illustration: Figure 10.--CALENDRICAL GEARING DESIGNED BY AL-BIRUNI,
_ca._ A.D. 1000. The gear train count is 40-10+7-59+19-59+24-48. The
gear of 48 therefore makes 19 (annual) rotations while that of 19-59
shows 118 double lunations of 29+30=59 days. The gear of 40 shows a
(lunar) rotation in exactly 28 days, and the center pinions 7+10 rotate
in exactly one week. After Wiedemann (see footnote 20).]
The phrase "resembled internally" is of especial interest in this
passage; it may perhaps arise as a mistranslation of the technical term
for stereographic projection of the sphere, and if so the device might
have been an anaphoric clock or some other astrolabic device.
This is made more probable by the existence of a specifically Islamic
concentration on the astrolabe, and on its planetary companion
instrument, the equatorium, as devices for mechanizing computation by
use of geometrical analogues. The ordinary planispheric astrolabe, of
course, was known in Islam from its first days until almost the present
time. From the time of al-Biruni (_ca._ 1000)--significantly, perhaps,
he is well known for his travel account of India--there is remarkable
innovation.
Most cogent to our purpose is a text, described for the first time by
Wiedemann,[20] in which al-Biruni explains how a special train of
gearing may be used to show the revolutions of the sun and moon at their
relative rates and to demonstrate the changing phase of the moon,
features of fundamental importance in the Islamic (lunar) calendrical
system. This device necessarily uses gear wheels with an odd number of
teeth (_e.g._, 7, 19, 59) as dictated by the astronomical constants
involved (see fig. 10). The teeth are shaped like equilateral triangles
and square shanks are used, exactly as with the Antikythera machine.
Horse-headed wedges are used for fixing; a tradition borrowed from the
horse-shaped _Farās_ used to fasten the traditional astrolabe. Of
special interest for us is the lunar phase diagram, which is just the
same in form and structure as the lunar volvelle that occurs later in
horology and is still so commonly found today, especially as a
decoration for the dial of grandfather clocks.
[Illustration: Figure 11.--GEARED ASTROLABE BY MUḤAMMAD B. ABĪ BAKR
OF ISFAHAN, A.D. 1221-1222. (_Photo courtesy of Science Museum,
London._)]
Biruni's calendrical machine is the earliest complicated geared device
on record and it is therefore all the more significant that it carries a
feature found in later clocks. From the manuscript description alone one
could not tell whether it was designed for automatic action or merely to
be turned by hand. Fortunately this point is made clear by the most
happy survival of an intact specimen of this very device, without doubt
the oldest geared machine in existence in a complete state.
[Illustration: Figure 12.--GEARING FROM ASTROLABE SHOWN IN FIGURE 11.
The gear train count is as follows: 48-13+8-64+64-64+10-60. The pinion
of 8 has been incorrectly replaced by a more modern pinion of 10. The
gear of 48 should make 13 (lunar) rotations while the double gear of
64+64 makes 6 revolutions of double months (of 29-30 days) and the gear
of 60 makes a single turn in the hegiral year of 354 days. (_Photo
courtesy of Science Museum, London._)]
This landmark in the history of science and technology is now preserved
at the Museum of the History of Science, Oxford, England.[21] It is an
astrolabe, dated 1221-22 and signed by the maker, Muḥammad b. Abī
Bakr (died 1231-32) of Isfahan, Persia (see figs. 11 and 12). The very
close resemblance to the design of Biruni is quite apparent, though the
gearing has been simplified very cleverly so that only one wheel has an
odd number of teeth (13), the rest being much easier to mark out
geometrically (_e.g._, 10, 48, 60, and 64 teeth). The lunar phase
volvelle can be seen through the circular opening at the back of the
astrolabe. It is quite certain that no automatic action is intended;
when the central pivot is turned, by hand, probably by using the
astrolabe rete as a "handle," the calendrical circles and the lunar
phase are moved accordingly. Using one turn for a day would be too slow
for useful re-setting of the instrument, in practice a turn corresponds
more nearly to an interval of one week.
[Illustration: Figure 13.--ASTROLABE CLOCK, REGULATED BY A MERCURY DRUM,
from the Alfonsine _Libros del saber_ (see footnote 22).]
In addition to this geared development of the astrolabe, the same period
in Islam brought forth a new device, the equatorium, a mechanical model
designed to simulate the geometrical constructions used for finding the
positions of the planets in Ptolemaic astronomy. The method may have
originated already in classical times, a simple device being described
by Proclus Diadochus (_ca._ 450), but the first general, though crude,
planetary equatorium seems to have been described by Abulcacim Abnacahm
(_ca._ 1025) in Granada; it has been handed down to us in the archaic
Castilian of the Alfonsine _Libros del saber_.[22] The sections of this
book, dealing with the _Laminas de las VII Planetas_, describe not only
this instrument but also the improved modification introduced by
Azarchiel (born _ca._ 1029, died _ca._ 1087).
No Islamic examples of the equatorium have survived, but from this
period onward, there appears to have been a long and active tradition of
them, and ultimately they were transmitted to the West, along with the
rest of the Alfonsine corpus. More important for our argument is that
they were the basis for the mechanized astronomical models of Richard of
Wallingford (_ca._ 1320) and probably others, and for the already
mentioned great astronomical clock of de Dondi. In fact, the complicated
gearwork and dials of de Dondi's clock constitute a series of equatoria,
mechanized in just the same way as the calendrical device described by
Biruni.
It is evident that we are coming nearer now to the beginning of the true
mechanical clock, and our last step, also from the Alfonsine corpus of
western Islam, provides us with an important link between the anaphoric
clock, the weight drive, and a most curious perpetual-motion device, the
mercury wheel, used as an escapement or regulator. The Alfonsine book on
clocks contains descriptions of five devices in all, four of them being
due to Isaac b. Sid (two sundials, an automaton water-clock and the
present mercury clock) and one to Samuel ha-Levi Adulafia (a candle
clock)--they were probably composed just before _ca._ 1276-77.
[Illustration: Figure 14.--ISLAMIC PERPETUAL MOTION WHEEL, after
manuscript cited by Schmeller (see footnote 26).]
The mercury clock of Isaac b. Sid consists of an astrolabe dial, rotated
as in the anaphoric clock, and fitted with 30 leaf-shaped gear teeth
(see fig. 13). These are driven by a pinion of 6 leaves mounted on a
horizontal axle (shown very diagrammatically in the illustration) and at
the other end of this axle is a wheel on which is mounted the special
mercury drum which is powered by a normal weight drive.
It is the mercury drum which forms the most novel feature of this
device; the fluid, constrained in 12 chambers so as to just fill 6 of
them, must slowly filter through small holes in the constraining walls.
In practice, of course, the top mercury surfaces will not be level, but
higher on the right so as to balance dynamically the moment of the
applied weight on its driven rope. This curious arrangement shows point
of resemblance to the Indian "mercury-holes," to the perpetual-motion
devices found in the medieval European tradition and also in the texts
associated with Riḍwān, which we shall next examine.
[Illustration: Figure 15.--ANOTHER PERPETUAL MOTION WHEEL, after the
text cited in figure 14.]
It is of the greatest interest to our theme that the Islamic
contributions to horology and perpetual motion seem to form a closely
knit corpus. A most important series of horological texts, including
those of Riḍwān and al-Jazarī, have been edited by Wiedemann
and Hauser.[23] Other Islamic texts give versions of the water clocks
and automata of Archimedes and of Hero and Philo of Alexandria.[24] In
at least three cases[25] these texts are found also associated with
texts describing perpetual-motion wheels and other hydraulic devices.
Three manuscripts of this type have been published in German translation
by Schmeller.[26] The devices include a many chambered wheel (see fig.
14) similar to the Alfonsine mercury "escapement," a wheel of slanting
tubes constructed like the noria (see fig. 15), wheels of weights
swinging on arms as described by Villard of Honnecourt, and a remarkable
device which seems to be the earliest known example of a weight drive.
This latter machine is a pump, in which a chain of buckets is used to
raise water by passing over a pulley which is geared to a drum powered
by a falling weight (see fig. 16); perhaps for balance, the whole
arrangement is made in duplicate with common axles for the corresponding
parts.
[Illustration: Figure 16.--ISLAMIC PUMP POWERED BY A WEIGHT DRIVE,
after the text cited in figure 14.]
The Islamic tradition of water clocks did not involve the use of gears,
though very occasionally a pair is used to turn power through an angle
when this is dictated by the use of a water wheel in the automata. In
the main, everything is worked by floats and strings or by hydraulic or
pneumatic forces, as in Heros devices. The automata are very elaborate,
with figures of men, monkeys, peacocks, etc., symbolizing the passage of
hours.
MEDIEVAL EUROPE
Echoes from nearly all the developments already noted from other parts
of the world are found to occur in medieval Europe, often coming
through channels of communication more precisely determinable than
those hitherto mentioned. Before the influx of Islamic learning at the
time of transmission of the Toledo Tables (12th century) and the
Alfonsine Tables (which reached Paris _ca._ 1292), there are occasional
references to the most primitive mechanized "visual aids" in astronomy.
The most famous of these occurs in an historical account by Richer of
Rheims about his teacher Gerbert (born 946, later Pope Sylvester II,
990-1003). Several instruments made by Gerbert are described in detail;
he includes a fine celestial globe made of wood covered with horsehide
and having the stars and lines painted in color, and an armillary sphere
having sighting tubes similar to those always found on Chinese
instruments but never on the Ptolemaic variety. Lastly, he cites "the
construction of a sphere, most suitable for recognizing the planets,"
but unfortunately it is not clear from the description whether or not
the model planets were actually to be animated mechanically. The text
runs:[27]
Within this oblique circle (the zodiac on the ecliptic of
the globe) he hung the circles of the wandering stars (the
planets) with marvellous ingenuity, whose orbits, heights
and even the distance from each other he demonstrated to
his pupils most effectually. Just how he accomplished this
it is unsuitable to enter into here because of its extent
lest we should appear to be wandering from our main theme.
Thus, although there is a hint of mechanical complexity, there is really
no justification for such an assumption; the description might well
imply only a zodiac band on which the orbits of the planets were
painted. On the other hand it is not inconceivable that Gerbert could
have learned something of Islamic and other extra-European traditions
during his period of study with the Bishop of Barcelona--a traveling
scholarship that seems to have had many repercussions on the whole field
of European scholarship.
Once the floodgates of Arabic learning were opened, a stream of
mechanized astronomical models poured into Europe. Astrolabes and
equatoria rapidly became very popular, mainly through the reason for
which they had been first devised, the avoidance of tedious written
computation. Many medieval astrolabes have survived, and at least three
medieval equatoria are known. Chaucer is well known for his treatise on
the astrolabe; a manuscript in Cambridge, containing a companion
treatise on the equatorium, has been tentatively suggested by the
present author as also being the work of Chaucer and the only piece
written in his own hand.
The geared astrolabe of al-Biruni is another type of protoclock to have
been transmitted. A specimen in the Science Museum, London,[28] though
unfortunately now incomplete, has a very sophistocated arrangement of
gears for moving pointers to indicate the correct relative positions and
movements of the sun and moon (see figs. 17 and 18). Like the earlier
Muslim example it contains wheels with odd numbers of gear teeth (14,
27, 39); however, the teeth are no longer equilateral in shape, but
approximate a more modern slightly rounded form. This example is French
and appears to date from _ca._ 1300. Another Gothic astrolabe with a
similar gear ring on the rete, said to date from _ca._ 1400 (it could
well be much earlier) is now in the Billmeier collection (London).[29]
Turning from the mechanized astrolabe to the mechanized equatorium, we
find the work of Richard of Wallingford (1292?-1336) of the greatest
interest as providing an immediate precursor to that of de Dondi. He
was the son of an ingenious blacksmith, making his way to Merton
College, Oxford, then the most active and original school of astronomy
in Europe, and winning later distinction as Abbot of St. Albans. A text
by him, dated 1326-27, described in detail the construction of a great
equatorium, more exact and much more elaborate than any that had gone
before.[30] Nevertheless it is evidently a normal manually operated
device like all the others. In addition to this instrument, Richard is
said to have constructed _ca._ 1320, a fine planetary clock for his
Abbey.[31] Bale, who seems to have seen it, regarded it as without rival
in Europe, and the greatest curiosity of his time. Unfortunately, the
issue was confused by Leland, who identified it as the Albion (_i.e._,
all-by one), the name Richard gives to his manual equatorium. This clock
was indeed so complex that Edward III censured the Abbot for spending so
much money on it, but Richard replied that after his death nobody would
be able to make such a thing again. He is said to have left a text
describing the construction of this clock, but the absence of such a
work has led many modern writers to support Leland's identification and
suppose that the device was not a mechanical clock.
[Illustration: Figure 17.--FRENCH GEARED ASTROLABE OF TREFOIL GOTHIC
DESIGN, _ca._ A.D. 1300. The gearing on the pointer is, from the
center: (32)/14-45+27-39, the last meshing with a concave annular gear
of 180 teeth around the rim of the rete of the astrolabe. A second
pointer, geared to this so as to follow the Moon, seems to be lacking.
(_Photo courtesy of Science Museum. London._)]
[Illustration: Figure 18.--GEAR TRAIN OF POINTER in figure 17. (_Photo
courtesy of Science Museum, London._)]
A corrective for this view is to be had from a St. Albans manuscript
(now at Gonville and Caius College, Cambridge) that described the
methods for setting out toothed wheels for an astronomical horologium
designed to show the motions of the planets. Although the manuscript
copy is to be dated _ca._ 1340, it clearly indicates that a geared
planetary device was known in St. Albans at an early date, and it is
reasonable to suppose that this was in fact the machine made by Richard
of Wallingford. Unfortunately the text does not appear to give any
relevant information about the presence of an escapement or any other
regulatory device, nor does it mention the source of power.[32] Now a
geared version of the Albion would appear to correspond very closely
indeed to the dial-work which forms the greater part of the de Dondi
clock, and for this reason we suggest now that the two clocks were very
closely related in other ways too. This, circumstantial though it be, is
evidence for thinking that the weight drive and some form of escapement
were known to Richard of Wallingford, _ca._ 1320. It would narrow the
gap between the clock and the protoclocks to less than half a century,
perhaps a single generation, in the interval _ca._ 1285-1320. In this
connection it may be of interest that Richard of Wallingford knew only
the Toledo tables corpus, that of the Alfonsine school did not arrive in
England until after his death.
There are, of course, many literary references to the water-clocks in
medieval literature. In fact most of these are from quotations which
have often been produced erroneously in the history of the mechanical
clock, thereby providing many misleading starts for that history, as
noted previously in the discussion of the horologium. There are however
enough mentions to make it certain that water clocks of some sort were
in use, especially for ecclesiastic purposes, from the end of the 12th
century onwards. Thus, Jocelin of Brakelond tells of a fire in the Abbey
Church of Bury St. Edmunds in the year 1198.[33] The relics would have
been destroyed during the night, but just at the crucial moment the
clock bell sounded for matins and the master of the vestry sounded the
alarm. On this "the young men amongst us ran to get water, some to the
well and others to the clock"--probably the sole occasion on which a
clock served as a fire hydrant.
It seems probable that some of these water clocks could have been simple
drip clepsydras, with perhaps a striking arrangement added. A most
fortunate discovery by Drover has now brought to light a manuscript
illumination that shows that these water clocks, at least by _ca,_ 1285,
had become more complex and were rather similar in appearance to the
Alfonsine mercury drum.[34] The illustration (fig. 19) is from a
moralized Bible written in northern France, and accompanies the passage
where King Hezekiah is given a sign by the Lord, the sun being moved
back ten steps of the clock. The picture clearly shows the central water
wheel and below it a dog's head spout gushing water into a bucket
supported by chains, with a (weight?) cord running behind. Above the
wheel is a carillon of bells, and to one side a rosette which might be a
fly or a model sun. The wheel appears to have 15 compartments, each with
a central hole (perhaps similar to that in the Alfonsine clock) and it
is supported on a square axle by a bracket, the axle being wedged in the
traditional fashion. The projections at the edge of the wheel might be
gear teeth, but more likely they are used only for tripping the striking
mechanism. If it were not for the running water spout it would be very
close to the Alfonsine model; but with this evidence it seems impossible
to arrive at a clear mechanical interpretation.
From the adjacent region there is another account of a striking water
clock, the evidence being inscriptions on slates, discovered in Villers
Abbey near Brussels;[35] these may be closely dated as 1267 or 1268 and
provide the remains of a memorandum for the sacrist and his assistants
in charge of the clock.
Always set the clock, however long you may delay on [the
letter "A"] afterwards you shall pour water from the
little pot (pottulo) that is there, into the reservoir
(cacabum) until it reaches the prescribed level, and you
must do the same when you set [the clock] after compline
so that you may sleep soundly.
A quite different sort of evidence is to be had from the writings of
Robertus Anglicus in 1271 where one gets the impression that just at
this time there was active interest in the attempt to make a
weight-driven anaphoric clock and to regulate its motion by some
unstated method so that it would keep time with the diurnal rotation of
the heavens:[36]
Nor it is possible for any clock to follow the judgment of
astronomy with complete accuracy. Yet clockmakers
(artifices horologiorum) are trying to make a wheel
(circulum) which will make one complete revolution for
every one of the equinoctial circle, but they cannot quite
perfect their work. But if they could, it would be a
really accurate clock (horologium verax valde) and worth
more than an astrolabe or other astronomical instrument
for reckoning the hours, if one knew how to do this
according to the method aforesaid. The method of making
such a clock would be this, that a man make a disc
(circulum) of uniform weight in every part so far as could
possibly be done. Then a lead weight should be hung from
the axis of that wheel (axi ipsius rote) and this weight
would move that wheel so that it would complete one
revolution from sunrise to sunrise, minus as much time as
about one degree rises according to an approximately
correct estimate. For from sunrise to sunrise, the whole
equinoctial rises, and about one degree more, through
which degree the sun moves against the motion of the
firmament in the course of a natural day. Moreover, this
could be done more accurately if an astrolabe were
constructed with a network on which the entire equinoctial
circle was divided up.
[Illustration: Figure 19.--MANUSCRIPT ILLUMINATION OF A MEDIEVAL
WATERCLOCK, showing a partitioned wheel, a weight drive, and a carillion
for striking. From Drover (see footnote 34).]
The text then continues with technical astronomical details of the
slight difference between the rate of rotation of the sun and of the
fixed stars (because of the annual rotation of the sun amongst the
stars) but it gives no indication of any regulatory device. Again it
should be noted, this source comes from France; Robertus, though of
English origin, apparently being then a lecturer either at the
University of Paris or at that of Montpellier. The date of this passage,
1271, has been taken as a _terminus post quem_ for the invention of the
mechanical clock. In the next section we shall describe the text of
Peter Peregrinus, very close to this in place and date, which describes
just such a machine, conflating it with accounts of an armillary sphere,
perpetual motion, and the magnetic compass--so bringing all these
threads together for the first time in Europe.
[Illustration: Figure 20.--ARRANGEMENT FOR TURNING A FIGURE OF AN ANGEL.
It has been alleged that this drawing by Villard represents an
escapement. After Lassus (see footnote 37).]
We have reserved to the last one section of evidence which may or may
not be misleading, the famous notebook of Villard (Wilars) of
Honnecourt, near Cambrai. The album, attributed to the period 1240-1251,
contains many drawings with short annotations, three of which are of
special interest to our investigations.[37] These comprise a steeplelike
structure labeled "cest li masons don orologe" (this is the house of a
clock), a device including a rope, wheel and axle (fig. 20), marked "par
chu fait om un angle tenir son doit ades vers le solel" (by this means
an angel is made to keep his finger directed towards the sun), and a
perpetual motion wheel which we shall reserve for later discussion.