Barometers and other mysteries.
For the first half of my career we used the beautiful and dangerous mercury barometers to measure atmospheric pressure. This coincided with a period of increasing use of radar altimeters by the military............ but I don't want to converge on airmanship so will let that rest.
Essentially, mercury was used because it is liquid and dense: much simpler and less prone to error than a column of water about 30 feet high. Thus the elegant [and poisonous] standard column of mercury which balanced atmospheric pressure and was attached to the Met. office wall near the observer. In old money [my currency] only about 30 inches of mercury are needed in the standard bore, and the calibration was in millibars [themselves now defunct], whereby the international standard for sea level was 1013.25 mb. So far, so good, but mercury expands with rising temperature and Met. offices are not often exactly at sea level. Corrections were needed.
Picture now the U Boat commander at the periscope ...... up periscope, push cap peak to rear, and crouch. Thus also the rookie observer at his new office, about to read the attached thermometer [see above] and then carefully read the height of mercury, allowing, as taught, for the meniscus. At which stage he is in the ideal posture to be goosed, and usually was. At Gutersloh, such was the hazard, that the standard tactic was to read with back to wall, and upside down.
Joking apart, pressure reading was emphasised as a very serious business: from a very early stage in training all observers were brainwashed to understand that one millibar error in reading or transmitting was about 26 feet of error on an altimeter AND COULD CAUSE A CRASH.
As a result of some serious errors [a little before my time] the Office instituted a very sensible running check board ........... before any mucking about with QFE QFF or QNH the observer was required to log his temperature-corrected hourly pressure reading in a time sequence on the observation desk. Errors of one mb or ten mb were commonest, and they stuck out like the scrotum on a Boxer dog. One mb in an hour was indeed possible, but only in rapidly changing weather, of which the observer was well aware ............ observations are not conducted in an information vacuum. So important was the matter regarded that the duty forecaster was required to check the check if flying was in progress.
Next: aneroid barometers and the barograph.