Early 19th century mahogany scientific barometer
by Negretti & Zambra, London
Common household stick barometers made with a fixed internal cistern size deliver readings with a small error due to an imbalance between cistern and tube volume. This imbalance affects readings exponentially as pressure reaches the extremes of the range, high and low. 
This had been a known problem for scientists since the late 17th century and several methods of implementing an adjustable cistern were tried over time, resulting finally in Nicholas Fortin's invention of the cistern system bearing his name in about 1800. Fortin's cistern was and still is used in most scientific mercury barometers.
Another scheme for adjusting, or "zeroing", cistern volume was developed by either Jesse Ramsden or George Adams in about 1770 (there is debate in the journals of the day for credit), using a gauge that incorporated a small plunger which floated from the surface of the mercury in the cistern. The fixed gauge and its movable plunger (right) have an index that can be brought into line by using the "portable screw" in the base of the barometer to push up or lower the leather diaphragm of the boxwood cistern to change the level of the mercury in the cistern, thus keeping cistern volume in step with that of the tube. This system does not adjust for elevation; its intent is just to correct the reading for volume.
This barometer was produced by Negretti and Zambra (1850-present day) at their first address of 11 Hatton Garden, London, between 1850 and 1859. The scales are engraved and the fine case is mahogany, with an old and very good finish. The original cistern and float system are still in place, and only the set knob is replaced. There is some minor restoration inside the cistern area, not visible when the cover is in place. The glass tube has a very large bore, common in scientific barometers to reduce the arc of the meniscus, and the tube seems to be original as well. It holds a large volume of mercury. The quality of the case and fittings are very high overall and the condition is excellent.
Circa 1850-1859
38" High
$1,900.00
No. 1677
