Bulgaria–United States relations to work. Bulgaria became a member of the Warsaw Pact on May 14, 1955. Though no Soviet troops were stationed on Bulgarian soil, in 1968, Zhivkov sent a Bulgarian division to join Polish, Hungarian and Soviet troops to crush a popular uprising in Prague. In 1978, the Bulgarian secret police were implicated in the assassination in London of Georgi Markov, a Bulgarian dissident who was a correspondent for the BBC World Service, Deutsche Welle radio and the U.S. sponsored Radio Free Europe. Markov was assassinated with a poisoned umbrella on September 7, Zhivkov's birthday. The rise of democratic movements across
Umbrella gas used by riot police. In the 1950s Frei Otto transformed the universally used individual umbrella into an item of lightweight architecture. He developed a new umbrella form, based on the minimum surface principle. The tension loaded membrane of the funnel-shaped umbrella is now stretched under the compression-loaded bars. This construction type made it technically and structurally possible to build very large convertible umbrellas. The first umbrellas of this kind (Federal Garden Exhibition, Kassel, 1955) were fixed, Frei Otto constructed the first convertible large umbrellas for the Federal Garden Exhibition in Cologne 1971. In 1978 he built a group of