368 



Hydrostatics. 



475. We have already mentioned, that, unless very particu- 

 lar precautions are taken, mercury is depressed in glass tubes, 

 and that this depression is inversely proportional to the diame- 

 ter of the tube. It is always indicated, moreover, when it takes 

 place by the upper surface being convex. It is not necessary 

 to have regard to this circumstance in the calculation of heights 

 by the barometer, where the two observations are taken with 

 the same instrument, since the difference in the length of the ba- 

 rometric columns would be the same, whether they were cor- 

 rected or not. t But in order that observations by different in- 

 struments, liable to different capillary effects, may be strictly 

 compared with each other, a correction should be applied, which 

 may be readily done by means of the following table. 



t Also in a syphon barometer, or one in which the tube, instead 

 of entering a basin, turns up at the bottom and continues of the same 

 bore, as in figures 230, 232, since the capillary effect is the same in 

 both branches, the observed altitude reckoned from the surface in 

 the shorter branch, would not be affected by the correction. 



