M. Blundevile his exercises containing sixe treatises, the titles wherof are set down in the next printed page: which treatises are verie necessarie to be read and learned of all yoong gentlemen that haue not bene exercised in such disciplines, and yet are desirous to haue knowledge as well in cosmographie, astronomie, and geographie, as also in the arte of navigation ... To the furtherance of which arte of navigation, the said M. Blundevile speciallie wrote the said treatises and of meere good will doth dedicate the same to all the young gentlemen of this realme.

How to know the true Longitude of any place. Chap. 9.

THough the Longitude may be found out by di∣uers waies not easie for euery mans capacitie, yet because Gemma Frisius thinketh none so sure as to knowe the same by the eclipse of the Moone, (which also as he saith) may sometime faile by reason of the diuersitie of aspects and Latitude of the Moone, and for that cause hath inuented a more readie way to finde out at all times the Longitude of any place, I minde here therefore briefely to shewe you first the order of finding out the Longitude by the Eclipse of the Moone, and then how to finde out the same by that readie way which he hath inuented: the order then to knowe it by the Eclipse of the Moone is thus: First you must learne by some Ephemerides at what houre the Eclipse shall be in some place, where you knowe alrea∣die by some Table the Longitude, that done, you your selfe or some other for you, must the same day of the Eclipse obserue by the Astrolabe at what houre the Eclipse beginneth in that place, whereof you knowe not the Longitude: For if the E∣clipse doe beginne in both places at one selfe houre, then assure your selfe that both places haue one selfe Longitude, but if it beginne sooner or later, then there is difference betwixt them, ac∣cording to the varietie of the time, which difference is thus to be knowne: Take the lesser summe of houres out of the grea∣ter, and there shall remaine either houres, or minutes, or both, if houres, then multiply the same by fiftéene, if minutes, deuide the same by foure, (for in this account fiftéene degrées doe make one houre, and foure minutes doe make one degrée) and adde the difference so found to the Longitude, if the Eclipse doe ap∣peare there sooner: if later then subtract the sayd difference from the knowne Longitude, and that which remaineth will shew the

vnknowne longitude. But note by the way, that if there remaine any minutes after the diuision, you must multiply those minutes by 15. and so shall you haue the minutes of degrees.

Shew the vse of this rule by some example.

For example I finde by the tables of Ptolomey that the longitude of Paris in France is 23. degrées, and by some Al∣manacke or Ephemerides I finde that the Eclipse doth begin there at thrée houres after midnight, now by this I would know the longitude of Tubing a famous citie in Sueuia, which is a region of Germany, at which towne in the verie day of the E∣clipse I cause to bee obserued by Astrolabe at what houre the Eclipse beginneth there, and I finde that it beginneth at 3. of the clocke and 24′· after midnight, then by subtracting the lesser number of time out of the greater, I find the remainder to bée 24′· which béeing deuided by 4′· which doe make one degrée, the quotient shall be 6. degrées, and that is the difference, which being added to the knowne longitude of Paris (because the E∣clipse is sooner there than at Tubing) it maketh in all 9. degrées, whereby I gather that the longitude of Tubing is 29. degrées, by this meanes all the Tables of Cosmographers are most com∣monlie made, and yet manie times they greatly differ in their longitudes for lacke perhaps of vsing diligence in taking the right houre and moment of the Eclipse, and for not dulie conside∣ring the diuers aspects, and what latitude the Moone hath at that instant which may cause great error.