BAROMETERS AND THE MEASUREMENT OF 

 ATMOSPHERIC PRESSURE 



CONTENTS 



Page 



Introductory— The barometer 1 



I. Mercurial barometer 2 



II. Aneroid barometers 21 



III. Miscellaneous barometers 25 



IV. Barographs 28 



V. Qeneralinstructions 45 



Page 

 VI. Concerning the elevation of station.s... 65 

 VII. Summary of special instructions for ob- 

 servers of the Weather Bureau 69 



VIII. Numerical tables 72 



Index 87 



introductory 

 The Barometer 



1. The hydrostatic principle, by virtue of which the pressure of the 

 air is measured by the ordinary barometer, was first formulated at 

 Florence in 1643 by Torricelli, whose famous experiments demon- 

 strated, not only that the air exerted a very great pressure, but that 

 this pressure changed slightly from day to day. 



2. TorricelWs barometer. — To repeat Torricelli's experiment, fill 

 a clean, dry, preferably warm, glass tube, closed at one end with pure, 

 dry mercury, using care to exclude all air. The length of the tube 

 must, in general, exceed 30 inches. Close the open end of the tube 

 firmly with the finger tip, and submerge it in an open cup of mer- 

 cury. Upon removing the finger and causing the tube to stand 

 vertically, a portion of the mercury will pass from the tube into the 

 cup, leaving a vacuous space, known as Torricelli's vacuum, in the 

 top of the tube. The column of mercury remaining in the tube will, 

 at sea-level stations, be about 30 inches high. The weight of this 

 mercury is sustained by and exactly balances the downward pressure 

 of the air upon the surface of the mercury in the cup. The height of 

 such a mercurial column, therefore, becomes a measure of the pressure 

 of the air, and Torricelli seems to have been the first to discover that 

 the height of such a column varied from day to day. 



3. Siphon tarometer. — Instead of constructing the barometer in the 

 manner just described, where the cistern and tube are in separate 

 parts, the tube may be made longer and turned up at the bottom so 

 as to resemble the letter J, forming what is commonly called a siphon 

 barometer, the long arm of which is closed at the top. 



4. Pressure of one atmosphere. — Suppose the area of the inside of 

 the barometer tube to be just 1 square inch, then a 30-inch barometric 

 column will contain just 30 cubic inches of mercury. Now, 1 cubic 

 inch of mercury weighs 0.4906 pounds, which, multiplied by 30, gives 

 the ordinary sea-level pressure of the air to be 14.718 pounds per 

 square inch. This quantity is frequently used by engineers, and is 

 called a pressure of one atmosphere. 



(1) 



