336 



CORRECTION FOR CAPILLARY ACTION. 



It is known that the effects of capillary action are not the same in different liquids. 

 In a tube plunged in water, the liquid in the tube rises higher than the level of the 

 water in the vessel, and terminates by a concave surface, which is called a concave 

 meniscus. In a tube plunged in mercury the liquid in the tube stands loioer than 

 the mercury in the vessel, and terminates by a convex surface, or a convex meniscus. 

 it is thus evident that the mercuri;d column in the tube of a Barometer does not rise 

 to its true height, and that it needs to be corrected for the depression due to capil- 

 larity, before it indicates the real pressure of the atmosphere. 



La Place, in the Mecanique Celeste., Tom. IV., has shown that the value of that cor- 

 rection depends upon the form of the meniscus, and gave a formula to compute it. 

 As this form varies in tubes of different bores, so does the depression, which dimin- 

 ishes as the diameter of the tube increases. The form of the meniscus, however, 

 was supposed to be the same in tubes of the same diameter, and constant in the same 

 tube ; and on this supposition the tables generally used for correcting the capillary 

 action have been computed. But more accurate observations have proved that, 

 owing to various causes not yet all well understood, the form of the meniscus is often 

 different in tubes of the same diameter, and that it is even variable in the tube of the 

 same instrument. 



It thus became necessary to construct new tables, taking into consideration, in a 

 given case, both the diameter of the tube and the form of the meniscus. Such tables, 

 with a double entry, have been given by Schleiermacher, in the Bibliotheque Univer- 

 seUe de Geneve, Tom. VIll. ; by Bravais, in the Annales de Physique el de Chimie, 

 Tom. V. p. 508 ; and by Delcros. The numbers in these tables agree very closely ; 

 but as Delcros's table is more extended than that of Schleiermacher, and in a more 

 convenient form than that of Bravais, it is given below, together with a reduction 

 of it to English measures, for the ordinary use. 



The other tables may serve for comparison. 



Table XXll., from the Report of the Committee of Phi/sics and Meteorology of 

 the Royal Society of London, 1840, gives the correction to be applied to English 

 barometers for capillary action in boiled and unboiled tubes. It lakes into account 

 the diameter of the tube, but not the variations of the height of the meniscus, or of 

 the convexity which terminates the barometrical column. This last element is sup- 

 posed to be in its normal state., and constant. 



Tables XXIII. and XXIV., by Delcros, in the Annuaire Meleorologique de France, 

 for 1849, give the means of finding the true correction to be applied to metrical 

 barometers for capillary action. 



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