"An Experiment on a Bird in the Air Pump" followed in 1768, the emotionally charged experiment contrasting with the orderly scene from "The Orrery". The painting, which measures 72 by 94½ inches (183 by 244 cm), shows a grey cockatiel fluttering in panic as the air is slowly withdrawn from the vessel by the pump. The witnesses display various emotions: one of the girls worriedly watches the fate of the bird, while the other is too upset to observe and is comforted by her father; two gentlemen (one of them dispassionately timing the experiment) and a boy look on with interest, while the young lovers to the left of the painting are absorbed only in each other. The scientist himself looks directly out of the picture, as if challenging the viewer to judge whether the pumping should continue, killing the bird, or whether the air should be replaced and the cockatiel saved.