Nick said:

>Note that 1 bar of pressure is equivalent to the pressure at the
>base of a 10m column of water. Coincidentally, the Earth's >atmosphere has a shielding effect equivalent to 10m of water
>(though the longer paths in the atmosphere have a better
>shielding effect from high energy cosmic rays).

Ten meters of water maybe too much water to have hanging over your head. Pure oxygen at 0.21 bar is not an option because of the fire hazard. Maybe a 50/50 mixture of oxygen/nitrogen would do the trick and halve that column of water. You don't want too much water over head because of the energy cost required in recirculating it. The glass has to be thick enough to drive a truck over. The ceiling would require a fair amount of maintenance, e.g. blowing off dust, scraping off algae, etc. Someone on a squba tank (or a robot) would have to swim between the panes of glass with a squeegee or a brush to deal with the algae. It'd be a bit like the guys who are constantly repainting the Golden Gate Bridge.

Gary