As the prayer times are determined according the sun’s position, the theory of sundials, how to draw and design them, hold a significant place in the history of Islamic astronomy. All Muslim scientists working on applied and theoretical sciences have books on this subject. Besides, as it was a part of madrasa curriculum, there are considerably high number of unexamined books from the beginning to the end of the Ottoman period. This study is about a manuscript titled Risālah-i Vaz‘-i Ruhāma, that we received from a second-hand book seller. Its anonymous author argues that he offers a simpler way to draw sundials designed for Istanbul. The author leaves the person willing to make a sundial free to determine the height of the gnomon and he shows a practical method to draw ruhāma with the help of goniometer and ruler prepared according to the determined height. Although the article presents the mathematical bases of the method, it is unable to verify clearly how the author derived the numeric values given in the book. To the end of our study, facsimile text and its simplified transcription are attached.