Geophysics, in the modern sense, started with geomagnetic works in the 1600s in the Ottoman Empire. The period between 1600 and 1800 included the measurement of magnetic declination, inclination and magnetic field strength. Before that time, there was only a little information available, such as how to use a compass, for example in the Kitabi Bahriye (the Book of Navigation) by Piri Reis, one of the most important mariners of the Ottoman Empire. However, this may not mean that magnetic declination was generally understood. The first Turkish scientific book relating to terrestrial magnetism was the book Fuyuzat-i Miknatissiye that was translated in 1731 from German into Turkish by Ibrahim Muteferrika. The subject of that book was earth's magnetism. The magnetic compass was mentioned in several books including Muhammed al Awfi's Jami al-Hikayat (translated into Turkish by Ibn Arabsah); Piri Reis's Kitab-I Bahriye (The Book of ` Navigation'); Seydi Ali Reis's Risale-i Mirat-I Kainat min Alat-I Irtifa (The Treatise called the Mirror of Universe according to the instrument for measuring Altitude) and Kitab Al-Muhit Fi. Ilm'al-Eflak Va'l Abhur (Book of the Regional Seas and the Science of Astronomy and Navigation). There were two original magnetic declination measurements made by Ottoman Turks in Istanbul in 1727 and 1893. Also, many geomagnetic measurements were carried out during international campaigns between 1600 and 1917 that visited Ottoman territory.