_Of the measuring of the earth._
Wee are now come to the last point concerning the measuring of
the Earth, which is two fold. Either of the
{ 1 Whole earth.
{
{ 2 Severall parts thereof, and their distance one from
{ another.
Concerning the first it is but a needlesse labour to recount the
diversity of opinions that haue beene held from time to time by
learned Geographers. What is the compasse and depth of the earth.
This may be seene in _Hues de vsu Globi, part. 3. cap. 2._ and in
_Clavius_ on _Sacrobosco_ with others. They all differ so much
one from another, that there is no certainty in trusting any of
them. The most common and received opinion is that the circuit of
the earth is 21600 miles, reckoning 60 miles for every degree,
and then the depth or Diameter of the Earth shall be 6877 English
miles, containing 5000 foote in a mile.
The means wherby the circuit and Diameter of the earth are found
out are principally two.
1 By measuring North or South, vnder one Meridian some good
quantity of ground, threescore or an hundred miles (or two for
the more certainty) for in those petty observations of small
distances there can be no certaine working. This may be done,
though it be laborious, yet exactly without any sensible error by
a skilfull workeman, plotting it out vpon his paper, with due
heed taken, that hee often rectifie the variation of the needle
(by which he travells) vpon due observation, and that all notable
ascents and descents, with such winding and turning as the
necessity of the way causeth, be reduced to one streight line. By
this means wee shall know how many miles in the Earth answering
to a degree in the Heauens; if exact observation by large
instruments be made to finde the elevation of the pole, in the
first place where wee begin to measure, and the last where wee
make an end.
Besides this way of measuring the circumference of the Earth,
there is none other that hath any certainty of observati[~o] in
it. That by Eclipses is most vncertain, for a little error in a
few minuts of time (which the observers shall not possibly
avoide) breeds a sensible and fowle error in the distance of the
two places of observation. That of _Eratosthenes_ by the Sunne
beames, and a shadow of a stile or gnomon set vpon the Earth, is
as bad as the other. For both the vncertainty of the calculation
in so small quantity as the shadow and the gnomon must needs
haue, and the difficulty to obserue the true length of the
shadow, as also the false supposition wherevpon it proceeds,
taking those lines for Paralells which are not, doe manifestly
shew the reckoning hereby made to be doubt full and not sure.
2 The second is by measuring the semidiameter of the Earth: For
as the circumference makes knowne the diameter, so doth this the
circumference. This may be done by observation made vpon some
great hill, hard by the sea side. The invention is of _Maurolycus
Abbot_ of _Messava_ in _Sicilie_, but it hath beene perfitted,
and more exactly performed by a worthy Mathematician _Ed. W._ who
himselfe made proofe of it. By this art was the semidiameter of
the Earth found out to be 18312621 foote: which allowing 5000
foot to a mile is 3662 & a halfe miles, which doubled is the
whole Diameter 7325 miles. The circuit of the earth shall be
23030 miles, and one degree containes 63-35/36 miles which is
almost 64 miles. Which as it exceeds the ordinary account, so may
wee rest vpon it as more exact then any other.
2 The second point concerninge the measuringe of particular
distances of places one from another is thus performed.
First vpon the Globe it is most easie. With a payre of Compasses
take the distance betweene any two places howsoever scituated
vpon the Globe, and apply the distance so taken to the Æquator, &
see how many degrees it takes vp; those degrees turned into miles
shew the distance of the two citties on from another.
Vpon vniuersall mapps theire is a little more difficulty in
finding the distance of places which here must bee considered in
a threefold difference of scituation: