Page:Madras Journal of Literature and Science, series 1, volume 6 (1837).djvu/79

Although this principle of specific gravity is necessarily concerned in the balance barometer, it will not prevent the tube descending further than a given distance, unless a certain relation exists amongst the portion of the apparatus concerned in that principle. Thus, if the tube be of a wide bore and very thin, so great a length of it would dip into the cistern, to displace a volume of mercury equal to what entered the tube by atmospheric pressure, that it might touch the bottom before the equilibrium were effected. Moreover, in the case of tube with expanded top, such as recommended in the original description, the tube increases in weight merely from descending, as this descent (as will afterwards be particularly shown) magnifies a portion of the narrow column of mercury, to the area of the large bore at top. It is not therefore to be inferred from the mere inspection of the plan, that this principle is the intended check to the unlimited descent of the tube. These remarks I consider it only justice to Mr. Taylor to make, because I have no doubt that, owing to my omitting to mention in the description of the plan that this is the check to the unlimited descent of the tube I had in view, he was induced to object to the balance portion of the apparatus. In the description of the plan (Madras Journal No. 14, page 27) I mentioned, that a balance, on the usual principles, will suit the purpose of the barometer; and the details of its construction there given, are, as there acknowledged, those recommended for delicate balances by Mr. Braddock, a gentleman to whose mechanical ingenuity and scientific attainments the pages of this Journal bear ample testimony. In such a balance, the centre of gravity is but a small remove below the fulcrum, so that with reference to Mr. Taylor's first diagram, the product of a very small additional weight (although much less would do, say 7 grains equal to 1/1000 of an inch of the mercurial column in the upper part of the barometer) into a e would exceed the product of the weight on the end b into b f, how low soever a descends; consequently, did no check to unlimited descent exist, the barometer end of the balance, always preponderating, would descend, and allow the tube to go down until it would reach the bottom of the cistern. In the absence of any mention of the required check it occurred to Mr. T. to place it in the balance; and, as the delicate one proposed, did not suit his views, he proposes to modify the balance in question, by placing the centre of gravity considerably below the fulcrum; in the particular case stated "about two inches." This ingenious idea of Mr. Taylor's would of course effect the object in view. The two objections which he mentions may be urged against it, do not appear of much importance, for with respect to the first, simply by moving a weight up and down a screw, may be effected the adjusting of "the axis of suspension to the height above the centre of gravity as shall render the scale we have adopted correct:" and with respect to