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This article raises the question of how oxygen levels have changed over the past 2 centuries, when carbon dioxide has been increasing.

John Mills
Decatur, Ala.

There is a problem in this interesting article. The graph of oxygen content versus time doesn’t agree with the text. Specific example: “About 255 million years ago … the oxygen concentration stood at 30 percent.” The graph shows this concentration at about 290 million years ago. Which is correct, graph or text?

Alan Sobel
Evanston, Ill.

I have long wondered about giant insects in the Carboniferous period. What about barometric pressure? Even with increased oxygen, would such insects be able to fly unless air pressure was higher as well?

Lindon Durvin
Richmond, Va.

Data gathered during the past decade show that the seasonal variations in the concentration of oxygen mirror those of carbon dioxide. Some discrepancies noted here arose because ancient concentrations of atmospheric oxygen have been estimated by various methods, including isotopic analyses of sediments. Also, the graph depicts new data that hadn’t been available to the other scientists cited in the text. Regarding insect flight, experiments with modern insects suggest that oxygen availability, not air density, is the factor that determines whether insects can fly .—S. Perkins