the second, which virtually is the necessity of having "recourse to computation," if this is to be considered an objection, what would become of Science—more especially of the department of which Mr. T. is the industrious cultivator? A more serious objection to it is, that it blunts the sensibility of the instrument, for, ceteris paribus, the lower the centre of gravity is placed below the fulcrum, the less delicate is the balance, and as this objection does not hold good with respect to the specific gravity check, it appears to me the more eligible one. In the weighing balance, the centre of gravity is placed below the fulcrum, to produce oscillation; also horizontality of beam, when equal weights are in the scale pans, but as oscillation is not required in the balance barometer the first variety of beam, named by Mr. Taylor, may be used, since the least additional weight moves it, while it is prevented upsetting by the dip or the immersion of the tube into or out of the mercury. In this variety of balance, having also the points of suspension in the same plane with the fulcrum, there is no tendency to move in any direction when equality of weight exists, but remains equally at an angle with the horizon as parallel to it; this therefore is exactly the balance wanted. If I understand Mr. Taylor aright, he considers objectionable the quantum of weight that will necessarily be on the balance, namely "tube of iron," column of "mercury," &c. It is no doubt true that the more a balance is loaded, it is the less delicate, but in the present case this appears more a theoretical than a practical objection, inasmuch as "in Europe, where every obstacle in the shape of workmanship vanishes," balances are made, which, when loaded much heavier than what the balance of the barometer requires to be, turn with much less than 7 grains. A balance has been made of such nice sensibility, that, when loaded at each end with 250 pounds, it turned with one grain.[1]
As to the substance of which the tube consists, this necessarily requires two qualities amongst others: 1st, impermeability to air; 2d, the power of resisting the chemical action of mercury. Glass combines both of these qualities, and the only objection to it is, its liability to fracture when boiling the mercury—of the metals, only two, so far as I am aware, are not acted on chemically by mercury—namely, iron and platinum. The former Mr. T. has shown to be objectionable, not being air tight. How is the latter circumstanced in this respect? If some of the other metals are impermeable to air, would it not suit to plate with the most eligible one so much of the tube as will not dip into the cistern?
I proceed to mention what appear improvements on the plan as originally proposed—1st With respect to the register apparatus.—It is advisable to remove the counterpoise weights UUU (Madras Journal
- ↑ This balance was constructed on the plan and under the direction of the late Captain Kater, V.P.R.S. for verifying the national standard bushel—vide Phil. Trans. 1826.
