NEVER ship a mercury barometer by post unless you drain it of all its mercury first. Like all the airlines, Royal Mail prohibits mercury as a "corrosive substance" but even if it was safe, the instrument would leak and would not work on arrival. So never buy a mercury barometer unless you can collect it in person or the seller is happy to deliver it.
More important, mercury is damaging to human tissue so always wear a pair of latex gloves for protection. Also, remove your jewellery because mercury also dissolves gold and silver!
The safest way to transport a mercury STICK barometer (see Image 1), is upside down but only if it has a sealed cistern so don't try this with the similar-looking Fitzroy barometer. Underneath the bottom of the instrument you'll find a transit knob. There are various types (sometimes you'll need a small clock key) but the purpose is to fill the mercury tube to the top. Hold a plastic bag underneath to catch any leaks and turn the knob/key. This squeezes the leather in the bottom of the boxwood cistern (Image 2) and forces the mercury in the reservoir up the tube. Watch the mercury level rise as you turn the knob/key and STOP when it reaches the top. (If you continue turning the knob, the old leather diaphragm in the cistern will split and all the mercury will escape). Now lift the instrument off the wall and slowly invert it. It's now ready to transport carefully by hand inside a plastic bin liner.
On arrival and before you go indoors, slowly turn the instrument the right way up again keeping the bin liner at the ready, just in case. Check the mercury is still full to the top of the tube - if it's not, some must have leaked so stop, place it in the bin liner and take advice. Assuming all is well, however, hang it on the wall where it is to be displayed, and let it settle for a minute. Then slowly undo the transit knob (with the bin liner still below it for safety) and watch the mercury column start to fall. After a few turns the column of mercury will stop dropping - it has now reached its working height but continue unscrewing the transit knob two or three more turns; contrary to popular belief, the knob is NOT an adjuster to calibrate the barometer.
Mercury wheel barometers are much more difficult to prepare for transport and really require two people. First, you will need a transit plug, made from a 150mm length of stainless steel with a small (size 1) cork in the middle and cotton wadding wrapped around the sharp end (Image 3). You can make one if there isn't one in the back already, or PayPal me £6.00 with a request and your address, and I'll post you one.
Gently lift the instrument down from the wall and place it in a plastic bin liner in a plastic tray. Lean it forwards about 30 degrees to cause the mercury column to rise - you will see the pointer on the dial move clockwise and you will hear or feel a feint click; this is the mercury reaching the top of the tube inside and this minimises the risk of spillage from the open end. Carry the instrument outside (still in the bin liner and tray) so that if there is any spillage, the fumes quickly dissipate. Next, still leaning it forwards, open the long thin back panel to expose the tube and the brass pulley system. Pin a card or something just below the pulley to hold the two silk threads tight on the double pulley wheel. Then lift out the left glass float inside the U-bend of the main tube and let it hang loose. Place the transport plug in its place until the cotton padding plugs the narrow opening near the bottom of the tube. Then slide the cork down and push it gently into the wider neck of the tube to hold it in position (Image 4). Close the door carefully, so as not to damage the glass float and now you can carry the instrument but always keep it in the bin liner and at an angle to ensure the mercury remains full in the top of the tube.
If travelling by car, have someone hold it at the same angle or secure it somehow so that it cannot shift. Once you arrive (still leaning the barometer forwards), open the back, remove the cork and pull out the transit plug, lower the glass float back into the tube and remove the card. Close the door and hang it up.
Finally, aneroid barometers (see Image 5) are much simpler to transport - there are no harmful contents and no spillage risk so you can hold them in any position. You can even post them so long as you're sensible with packaging and allow plenty of space in a large strong box filled with polystyrene chips. If moving them abroad, send them by surface mail as damage might result if shipped by air because of much lower external pressures at high altitude.
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