ry quarter of an
hour. The punches, belonging to the fifth, are marked on the said
scrowl, by the revolutions of the vane, which are accounted by a
small numerator, standing at the top of the clock-case, which is
moved by the vane-mill.

What, exactly, were the instruments applied by Hooke to his weather
clock? It is not always easy even to guess, because it appears that Wren
was actually the first to contrive such a device and seems to have
developed nearly as many instruments as Hooke. It might be supposed that
Hooke would have adapted to the weather clock his wheel-barometer,
introduced in 1667, but it also appears that Wren had described (and
perhaps built) a balance barometer before 1667.[10] As to the
thermometer, we have no evidence of original work by Hooke, but we do
have a description of Wren's self-registering thermometer, a circular,
mercury-filled tube in which changes in temperature move "the whole
instrument, like a wheel on its axis."[11]

The hygroscope (hygrometer) probably existed in more versions than any
other instrument, although we know nothing of any versions by Wren.
Hooke may have used his own "oat-beard" instrument.[12] Derham follows
his description of the clock--which has been quoted above--with a
detailed description of a tipping-bucket rain gauge invented by Hooke
and used with the clock. He also notes that in 1670 Hooke had described
two other types of rain gauge in which a bucket was counterbalanced in
one case by a string of bullets and in another by an immersed weight.
But here again, Sprat records the invention of a tipping-bucket gauge by
Wren before 1667.

Hooke has been generally regarded as the first inventor of an
anemometer, in 1662.[13] But this invention was a pressure-plate
gauge--that is, a metal plate held with its face against the
wind--whereas the gauge used with the weather clock is clearly a
windmill type, of which type this may be the first. Wren also had an
anemometer, but we have no description of it. Hooke's account does not
refer to other instruments which the weather clock is supposed to have
had, according to a description quoted by Gunther, which concludes the
enumeration of the elements recorded with "sunshine, etc."[14] One can
only wish for further information on the mechanism by which the
punches--or in Wren's clock, the pencils--were moved. But it is apparent
that Hooke's clock was actually used for some time.

[Illustration: Figure