the magazine Popular Mechanics of one of his "Dean drive" devices. The witness reported that "While suspended above the ground, was able to pull a load to itself without itself being pulled toward the load". Another version of the machine was reported to be "able to apply a force to a hand, without moving—yet when the machine was turned off an equivalent force applied by the hand easily moved the machine". William O. Davis, who witnessed the latter demonstration, wrote in his notebook about Dean's explanation of how the device worked, "... does not strike me as valid ... For this reason I have decided to undertake a theoretical study of dynamic systems to see if a concept can be evolved which will describe a world in which Dean's Drive can exist and yet where other known facts are not contradicted." Davis produced a hypothesis and it was published in Analog in 1962. Later analysis has revealed that the interactions of vibration, friction, and resonance with the springs of the scale are likely the root cause of the apparent weight loss reported by Campbell and others of apparent "anti-gravity" and "reactionless thruster" effects. Research conducted in conjunction with academia and industry has resulted in analytical equations identifying initial and rotational speed of the eccentric masses of the mechanism as key parameters of the inertial effects of the drive. An electromagnetic analog of the drive was also introduced. Similar results hold for other eccentric profiles. Interested parties In the 1950s Jerry Pournelle, working for an aerospace company, contacted Dean to investigate purchasing the device. Dean refused to demonstrate the device without pre-payment and promise of a Nobel prize. Pournelle's company were unwilling to pay for the right to examine the device and never saw the purported model. 3M sent representatives about the same time, and obtained similar results. Pournelle eventually was convinced that Dean's device never worked. Patents Dean was granted two U.S. patents on mechanical devices presumably related to his Dean drive claims. His first patent for a "System for converting rotary motion into unidirectional motion" was granted on May 9, 1959. Another patent for a "Variable oscillator system" was granted on May 11, 1965. These two patent designs were the only ones where Dean revealed the design details with an explanation for operation. However, the details were incomplete, and it is not possible to build a Dean Drive just