MY colleague Gerald Fleming, who presents the weather forecast on TV, receives many letters from devoted fans. Recently one gave details of an "old country idea for a very crude barometer", assembled in the following way.
"Put about an inch of water in a 11b jam jar or a similar vessel. Then using a long necked bottle (the half brandy bottle is ideal), insert the neck until it just touches the water, with the shoulder of the bottle resting on the top of the jar.
The amount of water will depend on the length of bottleneck used." The writer went on to tell Gerald that "I have followed it closely with your weather forecasts for the past three to four months, and it is remarkably close to the patterns in your diagrams. During the high pressure the water rises over three inches in the neck of the bottle, and falls right down during a depression."
Mercury barometers are made by filling a glass tube about 40 inches long with the liquid, and then inserting the tube into a dish containing more mercury. This done, the fluid in the now inserted tube falls far enough to leave a standing column about 30 inches high but no further. Since the space above the mercury column has had no contact with the outside air throughout the operation, it is as near a vacuum as makes no difference. Clearly, it is the atmospheric pressure pressing down on the external mercury that keeps the liquid inside the tube in suspended in animation.
Of course, a similar instrument can be made with water as the working fluid, but since water is only one thirteenth the weight of mercury, a much greater "head" is required to balance the pressure of the outside atmosphere, a tube 30 ft in length is needed to achieve the desired effect.
In the case of the device described by Gerald Fleming's correspondent however, the bottle still contains air, this presses downwards on the water in the neck, more or less balancing the pressure of the outside atmosphere and allowing the water column to move up or down by only an inch or two.
Because of this complication, the device is of little use for accurate measurements of barometric pressure. But it does react to pressure changes in its own way. As the writer put it. "While it did not always tell us when we could expect completely dry weather or the reverse, it did seem to indicate changes in pressure, and these more or less reflected the weather pattern. But the odd shower when making hay `with the bottle up', as the old folks used to say, often caused the thing to be branded useless."