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SCIENCE 601
London, Canterbury, Paris, and other places at the end ofthe
thirteenth century, and in Milan, St. Albans, Glastonbury,
Avignon, and elsewhere in the early fourteenth century. But
the earliest true clocks of which the mechanism is definitely
known are the Dover castle clock, which is now in the Science
Museum in London and used to be dated 1348 but is almost
certainly later, and Henri de Vick's clock set up on the Palais
Royal in Paris in 1 370.
Clocks may be said to have introduced the ordinary man to
the notion of mathematical time, divided into equal and in"
different intervals; mathematical space, extended into three
dimensions in equal units of length, was made the measure of
his world by the cartographer. As the natural and liturgical
seasons gave way in the organization of time to a mechanical
measure, so, alongside the hieratic maps,
like the Hereford
Mappa mundi of 1 3 14, depicting the world divided into regions
according to their spiritual relation with the holy city ofJeru' salem, there were made maps by which travellers and mariners
could find their way over the surface ofthe visible globe. Ofthe
most accurate ofthese medieval maps, the portolani
or compass^
charts, made and used by mariners in conjunction with a conv
pass, there are no known English examples; but two maps of
England are pioneer ventures in mapping on land and show
that progress was made. The first was drawn by Matthew Paris
about 1250 (PL 130), and the second, the so-called *Gough
map*, was drawn by an unknown cartographer somewhat less
than a century later (PL 21 in Chap. VI). Both show roads
and towns, but the second is much more accurate, and also
indicates mileages, probably as estimated by travellers. Roger
Bacon belongs also to the history of English cartography, not
only for his recognition of the need for accurate astronomical
measures of latitude and longitude, but also for a pregnant
mistake. His belief that there was no great width of ocean
between Europe and China became known to Columbus
through the writings of Pierre D'Ailly and Aeneas Sylvius;
it is said to have encouraged him to make the voyage by which
he discovered the New World.