19th Century Antique English Georg III Mahogany Barometer

The banjo barometer is also known as the wheel barometer. It was invented by Robert Hooke in 1663. He notes that the barometer may forecast and forewarns on the weather thereby preventing many dangers, and the good of mankind thereby promoted. He notes that he states that he founds most certain in predicting rainy and cloudy weather when it falls low, and clear weather when it rises high.

Furthermore, unlike its predecessor the Torricellian tube, the banjo is a siphon barometer in which the tube takes is in the form of a U. If the two mercury surfaces are equal in the area in such a barometer, a given change in height of the mercury column will be divided equally between the two ends, so that the movement of either surface will be about half as much of the mercury in the Torricellian tube. To magnify the motion of the mercury, the barometer has a large bulb on the upper surface hence concentrating motion in the lower exposed arm. A float resting on the exposed mercury surface in the open arm and a line from it passed over the pulley and ended in a counterweight. The pointer on the shaft of the pulley moves over a scale greatly magnifying the mercury motion and consequently better readouts.