The Sensex at the Bombay Stock Exchange declined 50 points, or 0.3 per cent, to 17,878, amid weak European bourses, today. Earlier in the session, the Sensex had hit a 30-month high of 18,040 points, but could not sustain those gains.The Nifty at the National Stock Exchange fell 18 points, or 0.3 percent, to 5,368. But stock markets in China, Hong Kong, Singapore and South Korea gained between 0.1 percent and 2 percent. MM/1705 hrs