In the Great Exhibition Building of 1851 was a colossal Barometer, the
tube and scale reaching from the floor of the gallery nearly to the top
of the building, and the rise and fall of the indicating fluid being
marked by feet instead of by tenths of inches. The column of mercury,
supported by the pressure of the atmosphere, communicated with a
perpendicular tube of smaller bore, which contained a coloured fluid
much lighter than mercury. When a diminution of atmospheric pressure
occurred, the mercury in the large tube descended, and by its fall
forced up the coloured fluid in the smaller tube; the fall of the one
being indicated in a magnified ratio by the rise in the other.