The Air Mill is the rotational analogue of the Guinea and Feather Tube. The mill is placed under the bell jar of a vacuum pump. One set of blades is oriented broadside to the rotation to provide the maximum amount of air resistance, while the other set is turned edge-on to give the minimal resistance. The two sets of vanes can rotate independently of each other. Initially the system contains air, and the vanes are set in motion by raising the sliding weight at the bottom of the apparatus with a rod passing through the sliding vacuum seal at the top of the bell jar. When the weight is released, it falls, and the rack attached to it engages pinions attached to the axles carrying the vanes, making them spin. The rack is short enough to permit the wheels to spin freely when the weight is in its lowest position. The vanes rotating edge-on continue to spin for some time, while the other set comes to rest rapidly because of air resistance.

   The receiver is then partly exhausted, and the experiment repeated, with the result that the broadside-on vanes still take a longer time to come to rest, although the difference in stopping times is reduced. Repetition of the experiment with increasing exhaustion of the bell jar will soon convince the observer that both wheels would spin for the same length of time in the total absence of air. When the experiment was over and the bell jar fully evacuated, air was let back into the receiver through a hole in the base plate, and the stream of returning air struck one side of the vanes and caused them to spin.
 

This air mill is in the Millington/Barnard Collection at the University Museum at the University of Mississippi in Oxford. Although it is unmarked, it may be by Lerebours et Secretan of Paris, from whom Frederick A.P. Barnard bought a good deal of apparatus in the second half of the 1850s. If so, its cost, exclusive of shipping across the Atlantic and up the Mississippi River, was 40 francs, or about $8.

REF: Thomas B. Greenslade, Jr., "Nineteenth Century Textbook Illustrations XLIII / The Air Mill", Phys. Teach., 22, 32-3 (1984)