The barometer known as the Fitzroy has one limb about 33in. to 30 in. long, the other limb being 2in. to 6 in. long. This tube is inclined and filled with boiled mercury, and on inverting it the mercury falls, leaving a vacuum several inches in length in the upper part of the long limb. There should be 2 in. or 3in. of mercury in the short limb to prevent air getting into the tube. The mercury rises and falls with the pressure of the air which is exerted on the surface of the mercury in the short limb, and in order that it may do this the short limb must be open or a hole must be blown in its side. The indicator is a metal pointer, which is moved to the upper surface of the mercury from day to day; it simply shows how the barometer stood the day before. Usually there is a dial on this form of barometer, and a pointer that moves round the dial; the pointer is actuated by a string and a weight in the shape of a glass rod, which rises and falls on the surface of the mercury in the shorter limb.