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 AEquator, & see how many degrees it takes vp; those degrees turned into miles shew the distance of the two citties on from another.