In preparing his maps, Mercator had to give attention to the study of the best methods of lettering. The results of these studies were published for the information of the public, in an essay on italic and cursive letters—"Ratio scribendarum literarum latinarum quas italicas cursoriasque vocant"—which was a treatise on calligraphy and much more; for it embodied the fruit of much thought and careful investigation on a subject which was of great importance, to him at least.
The services of Mercator were in frequent demand for the preparation of maps of private estates, and from this occupation he was able to meet the current expenses of his living and his family, and derived a considerable income.
Mercator's most important work, after his maps and atlases, was his chronology: "Chronologia a Mundi exordio ex eclipsibus et observationibus ac bibliis sacris" ("Chronology from the Beginning of the World; from Eclipses and Observations, and Holy Books"); Cologne, 1568; Basle, 1577. It was an elaborate work, the result of four years of labor, and gave 3965 years from the creation to the birth of Christ. Scaliger expressed a high opinion of it, and Lenglet Dufresnoy spoke of it as clear but dry; but it was pronounced by one of the best judges of the time, Onupbre Panvini, of Verona, author of several historical and chronological books, preferable to all existing chronologies. In the preface to this work he sketched a plan of a universal cosmography. Repeating this plan in 1585, he described it as intended to include, first, the form of the world and the general distribution of its parts; second, the order and motions of the heavenly bodies; third, their nature and radiation, and the concurrence of their influences, from which may be derived a veritable astrology; fourth, the elements; fifth, descriptions of kingdoms and of the whole earth; sixth, the genalogies of princes from the beginning of the world, with the emigrations of the peoples, their abodes, their first inventions, and antiquities. This order was not, however, observed in actual publication.
Piety was a predominant feature in all of Mercator's life. To extol the works of God, he said, "to exhibit the infinite divine wisdom and inexhaustible goodness by showing how all things in their composition concur to glorify him and reveal his incomprehensible providence—such is the end toward which I shall direct all my efforts, all my readings, and all my meditations."
This feeling it was, probably, that impelled him, in the latter part of his life, to give attention to theological questions, and which prompted the composition of his "Harmony of the Gospels" ("Harmbnia Evangelistarum" ), which was published at Duisburg in 1592.
He seems to have had opinions of his own on theological subjects, even earlier in his life—and that was against the order of society. He
