S66 Mathematics, 



on the subject, published in 1787.'' His plan is ta 

 obtain a measure of the greatest length that conve- 

 niency will permit, from two pendulums, whose 

 vibrations are in the ratio of 2 to 1, and whose 

 lengths coincide with the English standard in 

 whole 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 sur- 

 face 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, was first pro- 

 posed by Mr. Jefferson, now President of the 

 United States, in his report to Congress, on the 

 subject of w^eights and measures, while Secretary 

 of State, soon after the establishment of the fede- 

 ral government.'' It is supposed that this last men- 

 tioned standard is preferable, both in simplicity and 

 accuracy, to all others. 



The attempt made in France, within a few years 

 past, to form a standard of length, weight, and 

 capacity, by measuring a certain number of de- 

 grees on a great circle of the earth, is generally 

 known, This plan is considered, by good judges, 

 as having some important advantages; though in 

 simplicity and practicability it is certainly inferior 

 to the one last mentioned. 



The last century is also eminently distinguished 

 for the production of various kinds of Tables, which 

 are of the utmost importance to the mathematician, 

 particularly in giving facility and precision to his 

 calculations. Such of these as pertain immediately 

 to astronomy were mentioned in a former chapter. 



g An Attempt toivards obtaining invariable Measures of Length, Capacity 

 and fVeigf.'t, from the Mensuration of Time. 



r Mr. Jefferson does not claim the merit of this invention; it was 

 communicated to him by Mr. Robert Leslie, an ingenious watch-maker 

 of Philadelphia. 



