Captioned: "The mathematical jewel, shewing the making, and most excellent vse of a singuler instrument so called: in that it performeth with wonderfull dexteritie, whatsoeuer is to be done, either by quadrant, ship, circle, cylinder, ring, dyall, horoscope, astrolabe, sphere, globe, or any such like heretofore deuised. The vse of which iewel, is so aboundant and ample, that it leadeth any man practising thereon, the direct pathway through the whole artes of astronomy, cosmography, and briefely of whatsoeuer concerneth the globe or sphere: The most part newly founde out by the author, compiled and published by John Blagrave of Reading gentleman and well willer to the mathematickes, who hath cut all the prints or pictures of the whole worke with his owne hands. 1585." The mathematical jewel was an astrolabe he devised which had the benefit that it could be used anywhere in the world without the need to substitute different plates according to the latitude.