This book is an English translation of Book XVIII of the Astrologia Gallica, in which Morin explains in detail his theory of how to determine the strengths and weaknesses of the planets in a horoscope, a topic of great relevance and consequence to astrology. The present volume includes several horoscopes of notable persons that are used as examples of the rules. Morin was a well-known physician who became the most renowned of the French astrologers. He was appointed Royal Mathematician to King Louis XIII and was present at the birth of the future King Louis XIV. His massive Astrologia Gallica written in the scholarly Latin of the 17th century is now becoming available to English-speaking readers in a series of translations.