78 Mathematics. [Chap. VI. 



siderably improved upon thi^ idea, in his tract 

 on the subject, published in 17^7 *. His plan 

 is to obtain a measure of the greatest length 

 that conveniency will permit, from two pendu- 

 lums, of which the vibrations are in the ratio of. 2 

 to 1, and the lengths coincide with the English stan- 

 dard in ^\hoIe numbers. A further improvement 

 in the mode of employing this instrument as a 

 standard of measure is, to make use of a pendulum 

 without a bob, or a uniform cylindrical rod, which, 

 in a given latitude, at a certain height above the 

 surface of the ocean, and at a certain temperature, 

 shall vibrate in a given time. The use of a pen- 

 dulum of this kind, it is believed, Avas first proposed 

 by Mr. Jefferson, now president of the United 

 States, in his report to Congress, on the subject of 

 weights and measures, while secretary of state> 

 soon after the establishment of the federal govern- 

 ment f. It is supposed that this last-mentioned 

 standard is preferable, both in simplicity and accu- 

 racy, to all others. 



measuring the precise distance between the real point of suspen- 

 sion, and the centre of oscillation of the pendulum. 



To obviate this difficulty, Mr. Hatton, and after him, still 

 iiiore succcssfiill)', Mr. Whitehurst, devi«ed their improvements. 

 Since the publication of Air. Whitehurst, sir George Shuckburgh 

 Evelyn, asjsuming his principles, and pursuing his path, has made 

 further experiments, which throw new light on the subject. — Set' 

 rhilos. Trans. 1798, p. 174. 



*■ An Attempt tcivards obtaining invariable Measures of Lengthy 

 Capacity, and Weight, from the Mawiration of Time. 



t Mr. Jefferson does not claim tlie merit of this invention ; 

 it was communicated to him by Mr. Robert Leslie, an ingenious 

 watch-maker of rjj^^elphia. 



