which has a tube of the same shape as the wheel instrument, but of the same diameter from end to end except for a contraction at the bend. The reading of a siphon is the distance between the two surfaces of the mercury.
A VERY SIMPLE BAROMETER
is made by knocking off the neck of a small bottle, filling the body with water, and hanging it up by a string in the position shown (Fig. 154). When the atmospheric pressure falls, the water at the orifice bulges outwards; when it rises, the water retreats till its surface is slightly concave.