. Barometers and the measurement of atmospheric pressure. A pamphlet of information respecting the theory and construction of barometers in general. Barometers; Meteorological instruments. 8 19. Contracted haronneter scale.—As the correction for capacity in barometers with fixed cisterns remains the same so long as the quantity of mercury within the barometer and the inside area of tube and cistern are unchanged, it will not be necessary to apply a capacity correction to every reading made, provided we use on the barometer a scale having all its divisions shortened by just the proper amount to
. Barometers and the measurement of atmospheric pressure. A pamphlet of information respecting the theory and construction of barometers in general. Barometers; Meteorological instruments. 8 19. Contracted haronneter scale.—As the correction for capacity in barometers with fixed cisterns remains the same so long as the quantity of mercury within the barometer and the inside area of tube and cistern are unchanged, it will not be necessary to apply a capacity correction to every reading made, provided we use on the barometer a scale having all its divisions shortened by just the proper amount to compensate for the capacity effect. To understand this more clearly, imagine a barometer with the top of the column just 30 inches above the surface of the mercury in the cistern. Suppose the sectional area of the barometer tube at the top is only one-fiftieth as great as that of the cistern (this is about the usual relation). Now, if we imagine the column to rise a distance of 1 inch in the tube, it will then seem to become 31 inches high, but when the column rises 1 inch, the mercury in the cistern falls one-fiftieth of an inch, and, therefore, the real height of the column must be 31^ inches; that is, we may say, that each inch of a scale represents 1 finches of change in the real height of the mercurial column. If, therefore, a special scale be prepared having the spaces repre- senting inches, each one fifty-first part of an inch shorter than a true inch, then readings of our imaginary barometer on such a scale will indi- cate the true height of the column, presupposing, of course, that the sec- tional areas of the tube and cistern are uniform, and that the scale is ad- justed to a proper distance from the cistern. 20. By methods of calibration man- ufacturers are able to construct scales and barometers of great accuracy in accordance with the above principles, and they are very convenient to use. 21. It is obvious that if a barome- ter tube in such an instrument is brok
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