A briefe introduction to geography containing a description of the grounds, and generall part thereof, very necessary for young students in that science. VVritten by that learned man, Mr William Pemble, Master of Arts, of Magdalen Hall in Oxford.

CAP. 6.

Of the measuring of the earth.

VVEE are now come to the last point concerning the measuring the Earth, which is two fold, either of the

• W••••••e earth.
• 2 Severall parts thereof, and their di∣stance 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 ••ues de vsu Globe, part. 3. cap. 2. and in Clavi••s on Sacrobosco with others. They all differ so much one from another, that there is no certain∣ty in trusting any of them. The most common and received opinion is that the circuit of the earth is 21600 miles, reckoning 〈◊〉〈◊〉 miles for every degree, and then the depth or Diameter of the Earth shall be 6877 English miles, con∣taining 5000 foote in a mile.

Th•• means wherby the circuit and Diameter of the earth are found out are Principally two.

1 By measuring North or South, ••nder one Meridian some good quantity of ground, threescore or an hund••••d miles (or two for the more certainty) for in those petty ob∣servations of small distances, there can be no certaine work∣ing. 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 〈◊〉〈◊〉 often rectifie the variation of the needle (by which he travells) vpon due observation, and that all notable ascents and de∣scents, 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 an∣swering 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 obser∣atiō in it. That by Eclipses is most vncertain; for a little error in a few minuts of time (which the observers shall not possi∣bly avoide) breeds a sensible and fowle error in the distance of the two places of observation. That of Erat••sthenes 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 vncer∣tainty of the calculation in so small quantity as the shadow and the gnomon must needs haue, and the difficulty to ob∣serue the true length of the shadow, as also the false suppo∣sition wherevpon it proceeds, taking those lines for Parllells which are not, doe manifestly shew the reckoning hereby made to be doubtfull and not sure.

2 The second is by measuring the semidiameter of the Earth: For as the circumference makes knowne the diame∣ter, so doth this the circumference. This may be done by observ••tion made vpon some great hill, hard by the sea side. The invention is of Maurolycus Abbot of Mess••va in Si∣cili••, but it hath beene perfitted, and more exactly perfor∣med

by a worthy Mathematician Ed. W. who himselfe made proofe of it. By this art was the 〈…〉〈…〉idiameter of the Earth ••ound ou•• to be 1831262•• foote: which allowing 5000 foot to a mile is ••662 & a halfe miles, which doubled is the whole Diameter 7325 miles. The circuit of the earth shall be 2••030 miles, and one degree containes 63 61/36 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 par∣ticular distances of places one from another is thus perfor∣med.

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 di∣stance so taken to the Aequator, & see how many degrees it takes vp; those degrees turned into miles shew the di∣stance 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 conside∣red in a threefold difference of scituation:

• 1 Of Latitude only.
• 2 Of Longitude only.
• 3 Of Latitude and Longitude together.

1 If the two places differ only in Latitude, and lie vn∣der the same Meridian if the places lie both on one side of the Aequator, 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 di∣stance.

2 If the places differ only in Longitude, and lie both vnder one parallel of latitude the difference of longitude turned into miles proportionably accordinge to the latitude of the paralell, giues the true distance.

3 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 (A B C D) whereof (D) shall bee the Center. The greater this Semi-cir∣cle is made, so much the more easie will bee the operation; because the degrees will bee la〈…〉〈…〉. Then this Semicircle being drawne, and accordingly devided, imagine that by the helpe of it, you desire to find out the distance betwixt Lon∣don and Ierusalem, which Citties are knowne to differ both in longitude & latitude. Now, that the true distance betwixt th••se 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 vpō the Semi-circle, beginning at (A) & so proceed to (B;) & at the end of that difference, make a marke with the leter (ε) vnto which point by your ruler, let aright 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 ano∣ther 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 (ε), 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 ano∣ther perpendicular line that may fall with right angles vp∣on 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 (G H,)

and apply that widenesse vpon the longer perpendicular line (I K,) 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 sound to be 39 degrees: which being multiplied by 60. and so converted into miles accor∣ding to the former rules, will pro∣duce 2340. which is the distance of the said places.