1837.] 



Self -Registering Barometer. 



63 



Although this principle of specific gravity is necessarily concerned 

 in the balance barometer, it will not prevent the tube descending fur- 

 ther 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 ex- 

 panded 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, be- 

 cause 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 por- 

 tion 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 construc- 

 tion 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 j-qVo" °^ an ^ ncn °f tne mer - 

 curial column in the upper part of the barometer) into a e would exceed 

 the product of the w T eight on the end b into 6 /', 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 occur- 

 red to Mr. T. to place it in the balance ; and, as the delicate one pro- 

 posed, did not suit his views, he proposes to modify the balance in 

 question, by placing the centre of gravity considerably below the ful- 

 crum ; in the particular case stated " about two inches." This ingeni- 

 ous 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 mov- 

 ing a w 7 eight up and down a screw, may be effected the adjusting of 

 ,s 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 



