There are 2 different mercury barometers.
But the way you read the mercury barometers are for the stick type mercury barometer (you read the mercury height by looking directly at the top of the column inside the glass tube, and compare it to a scale of inches printed or engraved beside the column), and dial, also known as wheel or banjo (you read from a hand pointing to numbers on a dial.
Stick barometers: There is no physical alteration you can make to the barometer to correct its reading.
Not to any of them, even the ones with adjustable cistern capacities.
They are all made to operate at sea level, period.
At elevations up to 1,000 feet, just use the table below to make corrections to the reading on your stick barometer and enjoy it for the lovely antique it is.
Closing Screw At elevations above about 1,000 feet, the common household variety just flat won't work.
Short of cutting a section out of the case and shortening the glass tube as well, you can't make them read in synch with the corrected broadcasts.
Many owners mistakenly use the closing screw at the bottom of the case to push the mercury up to the "right" reading, a futile exercise.
Doing this just limits the capacity of the barometer's mercury system to accommodate high changes in volume.
You should be aware that many stick barometers are constructed with built-in errors in the relationship between the surface of the mercury in the cistern and the placement of the scale on the case.
Fitzroys are notorious for this; I've worked on dozens and I've yet to find an accurate one.
You can check yours with a tape measure.
Measure from any inches number on the register plate to the center of the cistern.
Twenty-nine inches on the scale should be placed 29 inches from the center of the cistern.29 Inches.
Dial barometers: With the more-or-less standard 35 inch long mercury tube that is generally used in most of these, there is latitude for some change to adjust your reading up to elevations of a maximum of 1,000 feet.
With the barometer hanging on the wall, remove the bezel that holds the glass over the dial.
Two or three screws around the perimeter are usually standard.
Take the barometer off the wall, keeping it upright, and open the back door.
Setting the hand Grasp the pulley in the back between thumb and forefinger with one hand, and with the other turn the indicator hand on its shaft to the right reading.
Put the barometer back in its normal position on the wall and check it.
A few adjustments may be required to get the right setting of the hand on the shaft.
If you don't feel comfortable doing this, don't try it.
There is considerable potential for messing up the workings of the instrument. Read our disclaimer.
If you reside above 1,000 feet and want your dial barometer to be accurate, a shorter mercury tube can be fabricated and installed.
We've done several and they have all worked, up to 6,800 feet in one case.
It isn't particularly expensive, and nothing is damaged by installing a shorter tube in the case.
The chances of the tube you have now of being original, or even over 100 years old, are slim to none.
Old glass corroded away in contact with mercury, and the tubes were replaced wholesale in order to keep the barometers working.
Yours has probably been done several times, so replacement has no effect on the antique value of the instrument as a whole.
In conclusion, a scientifically precise reading from your barometer doesn't really matter, in the grander scheme of things.
As long as the mercury level drops when a storm is coming and rises for fair weather, that is really enough of an indicator to make it a good predicting device.
That's all it did one or two hundred years ago, and it was good enough then.