The longest essay ever written is believed to be the one written by Ismail al-Jazari, a renowned Muslim inventor from the 13th century. The essay, titled Book of Knowledge of Ingenious Mechanical Devices, is estimated to contain over 100,000 words. In this work, al-Jazari described 50 inventions that he had made or was in the process of developing. His inventions included a variety of mechanical devices such as clocks and water pumps.

The essay includes detailed drawings of the different mechanisms as well as detailed explanations of how they work. Al-Jazaris essay also includes many mathematical equations to explain how his inventions function. It is believed that al-Jazari was inspired by the technology of other inventors and was able to build upon their work in order to create his own machines.

Al-Jazaris essay has become an important document in the history of engineering and technology. It is considered to be a groundbreaking work for its time and has been studied and admired by scholars for centuries. Al-Jazaris use of mathematics and ingenuity in creating his devices is still respected today and has served as an inspiration for many engineers and inventors.

The 'Book of Knowledge of Ingenious Mechanical Devices' is an impressive work that has withstood the test of time and continues to be admired today. Al-Jazari was an incredibly talented inventor who was able to create devices that are still relevant today. His essay remains the longest essay ever written, and it serves as proof of his impressive legacy.