On two successive days, I traveled on the London Underground and the Paris Métro. One contrast between the passengers struck me very forcibly. On the London Underground, not more than 5 percent of the passengers wore face masks. On the Paris Métro, not more than 1 percent did not. The contrast was all the more startling because the wearing of masks was equally obligatory on both systems. On the London Underground there were many notices saying so, and occasional public address announcements to that effect. On the Parisian Métro, there were likewise public address announcements, though pronounced much more clearly...