quickly they could put on a standard parachute at various g levels. The average times required by three subjects were 17, 21, and 41 seconds for g levels of 1.00, 1.41, and 2.24, respectively.

When the averaged data are plotted on a graph (see Figure 5) on the basis of the time required to complete a given action relative to the time at 1 g, it may be seen that the relative time depends on the type of action. However, it may be concluded that the work required to perform various acts becomes excessive above approximately 2 g.

Animal experiments have pointed to similar conclusions. At the University of California, Smith and others have grown chickens in centrifuges for extended periods of time (Tobias and Slater, 1962). These chickens were able to survive prolonged exposure to accelerations up to 4 g, but lost weight unless the acceleration was less than 2.5 g. In these gravitational fields, the heart rate increased and the rate of respiration decreased. The life span of small animals also appears to decrease at gravitational forces higher than 2 g. Some mice showed increased life spans at 1.5 and 2.0 £ (see Wunder et al., 1962). Both plants and insects apparently can tolerate extremely high g levels (thousands of g). It must be acknowledged, however, that centrifuge experiments, with their necessarily high and physiologically disturbing angular velocities, do not faithfully reproduce the linear gravitational field of a massive planet.

On the basis of the available data, one might conclude that few people would choose to live on a planet where the surface gravity was greater than 1.25 or 1.50 g. It is true that many people who are 25 to 50 per cent overweight live very normal lives and manage to accomplish as much as, or more than, many people whose weights conform more closely to the standards for their heights and ages. On the other hand, it is also generally true that physical activity is more exhausting to people who are carrying excessive burdens of fat. Overweight, of course, can not be equated with increased g on a one-to-one basis, but the relationship may give us a notion of the performance to be expected from human beings living on planets with gravitational accelerations greater than that of the Earth.

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