Safeguarding Subways (1 Letter) - NYTimes.com Re "Holding Back Floodwaters With a Balloon" (Nov. 20): Reading the article, I recognized an idea I'd seen before. United States Patent 940,323, "Bulkhead for Tunnels and the Like," was granted to George W. Jackson on Nov. 16, 1909. Mr. Jackson built Chicago's freight subway and needed a way to protect it from flooding. His patent covers a portable bulkhead with "a hollow flexible rim which is adapted to be expanded against the inner walls of the tunnel to form a watertight joint." Sadly, both the tunnels and Mr. Jackson's concern with flooding were forgotten by the time of the Chicago flood of 1992. His idea can certainly be applied to modern subway tunnels as well. Douglas W. Jones Iowa City