Posthuma Fosteri the description of a ruler, upon which is inscribed divers scales: and the uses thereof: invented and written by Mr. Samuel Foster, late professor of astronomie in Gresham-Colledg. By which the most usual propositions in astronomy, navigation, and dialling, are facily performed. Also, a further use of the said scales in deliniating of far declining dials; and of those that decline and recline, three severall wayes. With the deliniating of all horizontall dials, between 30 and 60 gr. of latitude, without drawing any lines but the houres themselves.

SECT. 2.

To finde how many Leagues doe answer to one degree of Longitude, in every severall Latitude.

THis (I say) may be done upon the Scales of Sines and equall parts: And for this purpose, the two last inches of the same Scale of equall parts, being equall in length to the Radius or Sine of 90, are di∣vided into 20 at one end, and into 60 at the other end.

Take therefore upon the line of Sines, the com∣plement of the parallels distance from the Equator, (or the complement of the given Latitude) and mea∣suring it upon the Scale of 20 parts, it will shew

you what number of Leagues make one degree of Longitude in that parallel of Latitude. And be∣ing measured upon the Scale of 60 parts, it gives so many of our miles, or so many minutes of the Equinoctiall, or any other great circle, as are answe∣rable to one degree of Longitude in that Latitude.

Example,

Let it be required to finde how many Leagues doe answer to one degree of Longitude, in the La∣titude of 18 gr. 12'.

Take out of the line of Sines, the complement of the given Latitude, namely. 71 gr. 48'. Then ap∣plying this distance to the Scale of 20 equall parts, you shall finde it to reach 19, and so many Leagues doe answer to one degree of Longitude, in the La∣titude of 18 gr. 12'.

And the same distance being measured upon the Scale of 60 equall parts, will give you 57 parts, and so many minutes of the Equator are answerable to one degree of Longitude, in that parallel of La∣titude.

So likewise, in the Latitude of 25 gr. 15', if you take the complement thereof 64 gr. 45', out of the Scale of Sines, and apply it to the former line of 20, you shall finde it to reach 18 parts, and so many Leagues doe answer to one degree of Longitude, in the Latitude of 25 gr. 15'.

¶In the Appendix to Master Norwoods Do∣ctrine of Triangles, there is by him laid

down 15 Questions of sailing by the plain Sea-chart, and others by Mercators Chart, all which the line of Chords and equall parts will sufficiently perform, if the work of the third Chapter of this Booke be rightly un∣derstood.