inch of the mercurial column, then follow these words " the surface cc rises to ee and consequently an additional weight (proportional 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 follows that a further rise of the surface ee will take place, and that, successively, cause and effect will ensue until the entire tube is filled, or the end of the tube 'brought up' by the bottom of the cistern." In this sentence I understand by the phrase "the whole tube bodily" the tube and its 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 obtain;—granting for a moment it does, I would arrive at a conclusion as to its effects, different from the one mentioned in the sentence quoted. What causes the tube in the first instance to commence descending? 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 occur, and of course a perpetual oscillation of the balance: it would appear, 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 surface 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
