1837.J 



Self- Registering Barometer. 



61 



inch of the mercurial column, then follow these words " the surface 

 " cc rises to ee and consequently an additional weight (proportion- 

 " al to ec) is imposed upon the barometer end of the balance ; now 

 " the effect of this weight being to lower the whole tube bodily, it fol- 

 " lows that a further rise of the surface ee will take place, and that, 

 " successively, cause and effect will ensue until the entire tube is fill— 

 " ed, or the end of the tube ' brought up' by the bottom of the cis-* 

 " tern." In this sentence I understand by the phrase " the whole tube 

 bodily" the tube audits contained mercury ; and by the phrase " further 

 rise" not a rise above 29.5 (the height to which by supposition the 

 mercury has by increased atmospheric pressure risen) but a second 

 rise, to that height, the mercury being considered to have fallen with 

 its containing tube, its top surface goes below 29.5, but remains below 

 it only temporarily, as the atmospheric pressure a second time forces 

 it up to that height. 



Presuming this to be Mr. Taylor's meaning, I am humbly of opinion 

 that this alternate rise and fall of the mercurial column, does not ob- 

 tain ; — granting for a moment it does, I would arrive at a conclusion 

 as to its effects, different from the one mentioned in the sentence quot- 

 ed. What causes the tube in the first instance to commence descend- 

 ing ? We both answer, the additional weight of half an inch added to the 

 mercurial column. If then the mercury with the tube descends half an 

 inch, it is evident that the mercurial column is shortened to its original 

 length, and consequently the additional half inch weight of mercury is 

 removed ; so that the tube, being thus freed of the weight which caused 

 it to descend, ought, I should think, instantly to rise, and remain up 

 until reloaded by a second rise of mercury. By this alternate loading 

 and unloading a perpetual alternate rise and fall of the tube would oc- 

 cur, and of course a perpetual oscillation of the balance : it would ap- 

 pear, however, Mr. Taylor considers that the tube thus lightened will 

 not re-ascend, but, by some means, not mentioned, it will remain down, 

 while the mercury makes a second rise. But that this alternate rise and 

 fall of the mercury does not occur, will, I think, appear evident when 

 we reflect that the mercurial column is supported by the atmosphere 

 and by that only — the tube merely prevents air reaching the top sur- 

 face of the mercury, and confines it to columnar limits — when the tube 

 is moved up and down it glides as it were over the mercury, exactly as 

 it would do over a rod of any solid substance, were this substituted for 

 the liquid metal. It is almost superfluous to add this has been proved 

 by experiment. 



It does at first sight appear somewhat incredible that the mercurial 

 column, in the case of increase of atmospheric pressure, should continue 

 rising ; while the tube containing it, when suspended from a balance, is 

 descending— for the ascent of the former and the descent of the latter 



