are simultaneous movements. But the enigma is solved the moment we comprehend that there are really two balances concerned, which have a common counterpoise—namely, the mercury. The one balance is composed of this and the atmosphere,—two fluids, the easy mobility of the particles of which allows a ready mutual counterpoise; the other balance is the visible one, of which the mercury and its containing tube at one end, and weight at the other, are counterpoises. Comprehending this, it is easy to trace what obtains, when an increase of atmospheric pressure occurs; the air forces into the tube mercury equal in quantity to its increased weight, and thus effects a counterpoise. But this increase of mercury destroys the equilibrium previously existing between it and the weight; the former preponderating, its end of the balance necessarily descends, but while the tube is descending, the mercury and air preserve their equilibrium, so that the former remains, in the case supposed, at 29.5 inches above the surface of that in the cistern, while the tube slides over it in the manner already described. It may be said that this explanation offers an objection to the balance barometer more serious than the one advanced by Mr. Taylor; inasmuch as his shows only an intermitting pull, while this shows a constant one. If no check to the unlimited descent of the tube existed, this would hold good; but let us now attend to what occurs to the lower portion of the tube. Here, as Mr. Taylor remarks, "it must be recollected that the part of the tube immersed in the quicksilver, lessens the weight imposed upon this end of the balance, to the amount of the weight of an equal volume of quicksilver." When, therefore, atmospherical pressure causes a given volume of mercury to enter the tube, this descends from increased weight, and necessarily dips into the mercury in the cistern: but this dipping causes the tube to lose so much of its weight as is equal to the volume of mercury it displaces;—so when the volume of mercury thus displaced equals that which entered the tube, an equilibrium is effected; the tube has lost the weight it gained; a further dipping would make it lighter than the counter-weights, because, the mercury remaining at 29.5, no additional weight is added by the column; while the further descent of the tube into the mercury makes it as much lighter as the volume of quicksilver it displaces; a further descent of the tube is thus prevented, and if it forced down further than the point at which this equilibrium occurs, it would rise to that point when the foreign force thus depressing it is removed. Here, then, in this principle of specific gravity, we observe a third balance concerned in the balance self-registering barometer; which, like the one consisting of the atmosphere and the mercury, is of the nicest sensibility; and the third balance discharges the important function of preventing the unlimited descent of the tube. The above are the reasons that induce me to think that the principle of my plan is correct.
