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 

 broken it will be difficult to find an- 

 other so nearly the same size that it 



could be used with the old scale ; generally a new scale is also required. 



22. In Figure 10 is shown a cut of an excellent barometer of the 

 fixed-cistern type, devised by Schneider Bros., of Jersey City, N. J. 

 One of the special features of the barometer is the means provided 

 for filling the cistern and tube with mercury so that the barometer 

 can be shipped safely from place to place. 



To fill the cistern with mercury, the barometer is first very care- 

 fully and gradually inclined and inverted. When fully inverted 



Figure 10. — Fixed cistern barometer 



