The structure of courses began to change in the first decades of the 18th century, when physical demonstrations were added to lectures. Pierre Polinière and Jacques Rohault were among the first individuals to provide demonstrations of physical principles in the classroom. Experiments ranged from swinging a bucket of water at the end of a rope, demonstrating that centrifugal force would hold the water in the bucket, to more impressive experiments involving the use of an air-pump.[8] One particularly dramatic air-pump demonstration involved placing an apple inside the glass receiver of the air-pump, and removing air until the resulting vacuum caused the apple to explode.[9] Polinière’s demonstrations were so impressive that he was granted an invitation to present his course to Louis XV in 1722.[10]