CAP. 6.

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.
  1. 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.
    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.
    2. Besides this way of measuring the circumference of the Earth, there is none other that hath any certainty of observatiõ 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.
    3. 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 means wherby the circuit and Diameter of the earth are found out are principally two.
    1. Of Latitude only.
    2. Of Longitude only.
    3. Of Latitude and Longitude together.
    4. If the two places differ only in Latitude, and lie vnder the same Meridian if the places lie both on one side of the Æquator, the differences of the latitudes: or the summe of both latitudes added together, if one place lie North and another South, being turned into Miles giues the true distance.
    5. If the places differ only in Longitude, and lie both vnder one paralell of latitude the difference of longitude turned into miles proportionably accordinge to the latitude of the paralell, giues the true distance.
    6. The distance of places differing both in latitude and longitude may thus bee found out, first let there bee drawne a semicircle vpon a right diameter noted with (ABCD) whereof (D) shall bee the Center. The greater this Semi-circle is made, so much the more easie will bee the operation; because the degrees will bee larger. Then this Semicircle being drawne, and accordingly devided, imagine that by the helpe of it, you desire to find out the distance betwixt London and Ierusalem, which Citties are knowne to differ both in longitude & latitude. Now, that the true distance betwixt these two places may be found out, you must first substract the lesser longitude out of the greater, so shall you find the differences of their longitudes, which is 47. degrees. Then reckon that difference vpõ the Semi-circle, beginning at (A) & so proceed to (B;) & at the end of that difference, make a marke with the leter (E) vnto which point by your ruler, let a right line be drawne from (D) the center of the Semi circle. This being in this sort performed, let the lesser latitude be sought out which in 32 degrees, in the fore said semicircle, beginning your accompt from the point (E) and so proceede towards (B), and at the end of the lesser latitude let another point be marked out with the letter (G), from which point, let there be drawen a perpendicular line which may fall with right Angles vpon the former line drawen from (D) to (E), and where it chanceth to fall, there marke out a point with the letter (H): This being performed let the greater latitude which is 51 degrees 32 minuts, be sought out in the semicircle beginning to reckon from (A) towards (B) and at the end of that latitude set another point signed out by the letter (I) from whence let there be drawen another perpendicular line that may fall with right angles vpon the diameter (AC): & here marke out a point with the letter (K), this done take with your compasse the distance betwixt (K) and (H) which distance you must set downe vpon the diameter (AC) placeing the one foot of your compasse vpon (K) and the other towards the center (D) and there marke out a point with the letter (L); then with your compasse take the shorter perpendicular line (GH,) and apply that widenesse vpon the longer perpendicular line (IK,) placing the one foote of your compasse at (I,) which is the bounds of the greater latitude, and extend the other towards (K), and there make a point at (M), then with your compasse take the distance betwixt (L) and (M), and apply the same to the semicircle. Placing the one foot of your compasse in (A) and the other towards (B), & there marke out a point with the letter (N), now the number of degrees comprehended betwixt (A) and (N) will expresse the true distance of the two places, which will bee found to be 39 degrees: which being multiplied by 60. and so converted into miles according to the former rules, will produce 2340. which is the distance of the said places.
  3. The second point concerninge the measuringe of particular distances of places one from another is thus performed.
  4. 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.
  5. 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: