i, 50-57, L. Wilkinson's
translation, revised by B[.a]p[.u] deva S(h)[.a]stri,
Calcutta, 1861.

Before proceeding to an investigation of the content of these texts it
is of considerable importance to establish dates for them, though there
are many difficulties in establishing any chronology for Hindu
astronomy. The _S[=u]rya Siddh[=a]nta_ is known to date, in its original
form, from the early Middle Ages, _ca._ 500. The section in question is
however quite evidently an interpolation from a later recension, most
probably that which established the complete text as it now stands; it
has been variously dated as _ca._ 1000 to _ca._ 1150 A.D. The date of
the _Siddh[=a]nta Siroma[n.]i_ is more certain for we know it was
written in about 1150 by Bh[=a]skara (born 1114). Thus both these
passages must have been written within a century of the great clock-tower
made by Su Sung. The technical details will lead us to suppose there is
more than a temporal connection.

We have already noted that the armillary spheres and celestial globes
described just before these extracts are more similar in design to
Chinese than to Ptolemaic practice. The mention of mercury and of sand
as alternatives to water for the clock's fluid is another feature very
prevalent in Chinese but absent in the Greek texts. Both texts seem
conscious of the complexity of these devices and there is a hint (it is
lost and revealed) that the story has been transmitted, only half
understood, from another age or culture. It should also be noted that
the mentions of cords and strings rather than gears, and the use of
spheres rather than planispheres would suggest we are dealing with
devices similar to the earliest Greek models rather than the later
devices, or with the Chinese practice.

A quite new and important note is injected by the passage from the
Bh[=a]skara text. Obviously intrusive in this astronomical text we have
the description of two "perpetual motion wheels" together with a third,
castigated by the author, which helps its perpetuity by letting water
flow from a reservoir by means of a syphon and drop into pots around the
circumference of the wheel. These seem to be the basis also, in the
extract from the _S[=u]rya Siddh[=a]nta_, of the "wonder-causing
instrument" to which mercury must be applied.

In the next sections we shall show that this idea of a perpetual motion
device occurs again in conjunction with astronomical models in Islam and