cell with very low diffusion through the walls and one capable of maintaining super-pressure in excess of that lost with reductions of gas temperature. Such a balloon could be sealed off completely or a pressure-activated valve could be used to permit efflux of the gas when the bursting pressure is approached. The neoprene-coated nylon balloon built by Seyfang Laboratories (Figure 7) has been used with a valve set to prevent rupturing.
Figure 7. Neoprene-coated nylon balloon, two-thirds inflated.
The fabric has been coated with a metallic paint to minimize the effects of radiation. However., the values of superheat obtained by the gas when the balloon is in the sun have been of the order of $ 3 0^{\circ} \mathrm{C} $ . The amount of buoyancy lost when circulation