The George II mahogany ‘sign post’ angle barometer that sold to a Derbyshire museum for £19,000 at Bamfords.

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Whitehurst, who made instruments for Matthew Boulton, pioneered both the wheel and angle barometer as well as making a variety of stick and other types.

The angle barometers were, however, his most accomplished pieces, and before the emergence of the present example, only 24 were known, just 11 of them dated. They are all in mahogany cases in two basic sizes, and the present example is of the smaller sort at 21in (53cm) high.

Whitehurst was a keen meteorologist and in the 1740s developed a 0-60 scale to express barometric pressure that he believed would be more practical than the usual 28-31 inch scale. It has been said that he invented the millibar a century and a half early - although ultimately it failed to catch on.

Whitehurst's nephew and successor continued to make barometers of this type for about a decade, before dropping them in favour of more conventional types.

This example, inscribed to the silvered register Whitehurst, Derby 1757, is marked with Whitehurst's unique 0-60 scale. Only two surviving examples of Whitehurst angle barometers bear earlier dates, one sold in New York in 1985 dated 1747 and one in a Derbyshire country house dated 1751.

The new discovery was found behind a wardrobe in a Derbyshire village and included in Bamfords' March 17-19 sale in Derby, estimated at £5000-8000.

Some restoration was needed to the crossbanded case but as an instrument it was deemed to be in wholly original condition. Interest arrived from many quarters including three museums - one local, one in London and one in the US.

When bidding reached £19,000 (plus 15 per cent buyer's premium) it sold to the Derbyshire museum who were underbid by a local collector.