Difficulties ef ohtain'mg a Meafurt of Length from a Pendulum* a^ 



VII. 



A popular Account of Experiments which have leen made or attempted for the Purpfe of obtaining 

 an invariable Meafure of Length, from the Difference between the Lengths of the fame Pen- 

 dulum, when adjujled to meafure different known Portiotis of Time. (IV^ N.J 



A, 



^BOUT the beginning of the prefent century, and for a-confiderable time afterwards, the-, 

 attention of fcientific men, namely Huyghens, Sir Chriftopher Wren, and others, was diredled 

 to the ufeful application of the pendulum, as an univerfal meafure capable of being obtained 

 from the menfuration of time, by any number of independent obfervers, who fhould carefully 

 follow the fame procefles of obfervation. Every one knows that the vibrations of a pendulum ■ 

 will be flower the greater its length; and that by the repeated meafurement of the fame portio.i-. 

 of time, the deviations from any given fradt-ion of a day, as for example a fecond, will be fhown 

 with extreme accuracy. Thus in the pendulum which Ihall vibrate feconds, the difference of 

 lefs than one-thoufandth of an inch in the length will be fliewn by a variation of one fecond 

 per day; and if the pendulum, intended to be ufed as a meafure, were adjufted fo as to keep 

 time without varying one fecond in the week, it Ihould feem to follow, that our general or 

 univerfal meafure would be obtainable to lefs than the feven-thoufandth part of an inch, or the ■ 

 two hundred-thoufandth part of the whole. But many difficulties prefent themfelves upon a. .. 

 nearer pra6lical view of the fubjeft. i. If the expanfion and contraflion from change of tem- . 

 perature be not provided for, we ftill have a caufe of error, which, in a fteel rod, would amount 

 to a fecond a day for every four degrees of the thermometer* ; and it fcarcely can happen in any 

 common fituation, that the thermometer will not vary many degrees in the . 24 hours. 2. 

 The wheel-work applied to keep account and to overcome the effedt of the affiftance of the air 

 will in moft, and, perhaps, in every cafe alter the meafure of the vibration . 3. If the point of fuf- 

 penfion have any fpring or mobility, the vibration will be flower than correfponds with the ap- 

 parent-length of the pendulum. Without entering into minute deduilions tefpefilhg this varia-. 

 tion and its quantity, according to different affumed laws, this effeft may be eafily fliewn by 

 hanging any weight to a firing, and holding it in the hand while it vibrates. The vibration' 

 will be much quicker if the hand be held flill, than if it be fuffered to follow the motion of the 

 pendulous body. 4. When a body moves in a right line, the whole of its motion may be confi- ' 

 dered as if it refided in.the centre of gravity ; but if it move in a curve, one part of the body will 

 move fwifter than the other, and the centre of motion, or place at which an obftacle would flop it 

 without occasioning rotation, will be nearer to the outer fide of the extreme curve than the centre, 

 of gravity itfelf. This centre in pendulums is called the centre of ofcillation. It is ufual in public 

 ledures to illuftrate this doflrine by means of a ftrait ftick. If the ftick be held by one end, and" 

 theobferver ftrikes an obftacle downwards with the middle or centre of gravity of the ftick, bis. 



• Philof. Journal, I. 58. 



hand 



