"What time is it?" - an easy question to answer nowadays when many people can just glance at their wrist or look at their computer screen - but less so in the past when far fewer people had time-pieces. Yet David Rooney, a former Curator of Timekeeping at the Royal Observatory, in Greenwich, told the fascinating story of the Belville family who - from 1836 to 1940 - sold the time to subscribers in central London, using an 18th Century pocket-watch, called Arnold.

In a brilliant talk, with serious science mixed with supreme comedic timing, David dwelt in particular on Ruth Belville, known as the Greenwich Time Lady, who travelled the streets of London for nearly fifty years with Arnold, which was set at Greenwich and accurate to within one tenth of a second.

Ruth's father, John Henry, an astronomer and meterorologist at the Royal Observatory, had started the business and his wife, Maria, has continued after his death in 1856, before stopping in 1892 - when Ruth continued selling the time until the start of World War II. Yet it was was amazing that Ruth, living in Maidenhead, kept going for so many years when she was already an anachronism - even back in 1908.

David pointed out that the Belvilles' main rival was not the Post Office but a commercial company, called the Standard Time Company (STC), which supplied the time electronically - yet Ruth's customers remained loyal. In fact, the STC's service was good but not always so; and accurate enough for most people most of the time - but not more so than Ruth's service with Arnold.

A measure of the regard in which Ruth was held is that, in 1941, she was awarded a pension of £15 a year by the trade guild, the Worshipful Company of Clockmakers, and it was doubled the following year. When she died in 1943, her faithful chronometer, made in 1794, had been used for 149 years - or over 4.7 thousand million seconds.

Interwoven into David's fascinating story of the Belville family were many other historic details, including France's attempt to have the meridian go through the Eiffel Tower and how they became bitter when the 1884 International Meridian Conference chose Greenwich for the prime meridian and universal time. Indeed, a French anarchist, Martial Bourdin, accidentally blew himself up when trying to plant a bomb at Greenwich.

Other fascinating strands included the competition in the 1930s for the first voice of Tim, the speaking clock. The judges, who included John Masefield, the poet laureate, and Dame Sybil Thorndike, stipulated that the winner should not have a regional voice. One of the losers was from Tyneside and, it emerged later, a distant relative of David's.

David had travelled form South Kenisngton, where he is now Curator of Transport at the Science Museum and his talk was an unmitigated triumph and everybody will surely remember it for a long time to come. The evening took place under the new lighting system and the next major phase of work is a new roof on the hall. Proceeds from the talk were £537, with David selling sold £220 worth of his book on Ruth Belville.

 What a magnificent talk! David Rooney brought humour and lucidity to a subject which, in less talented hands, could have seemed like rather dry science. At its heart was a remarkable story which he had unearthed and pursued with a passion and yet he told it in a way which left all of us wanting to hear more - the true yard-stick of a captivating talk. There is only one question, really - can David return soon to entertain us again? He has set the bar very high for the fourth season and he will be a hard act to follow - Colin Sumner, Oxford