LXII PHILOSOPHICAL SOCIETY OF WASHINGTON. 
don mint used in 1743 indicated one-eighth of a grain on a troy 
pound, or about one part in 50,000; while according to Sir George 
Shuckburgh the balance used by Messrs. Harris and Bird in mak- 
ing their observations upon the Exchequer weights, apparently in 
1758 or 1759, turned with one 230,000th part of its load. In 1798 
Sir George Shuckburgh had a balance sensitive enough to indicate 
0:01 of a grain when loaded with 16,000 grains, or about one part 
in 1,600,000. The balance used by Fortin in 1799, in adjusting 
the kilogram of the archives, was not quite so delicate, its sensi- 
tiveness being only the 1,000,000th part of its load; but in 1844, 
for the adjustment of the present English standard pound, Professor 
Miller employed a balance whose index moved about 0°01 of an 
inch for a change of 0°002 of a grain in a load of 7,000 grains” He 
read the index with a microscope, and found the probable error of 
a single comparison of two avoirdupois pounds to be one 12,000,000th 
of either, or about 0:00058 of a grain. At the present time it is 
claimed that two avoirdupois pounds can be compared with an 
error not exceeding 0°0002 of a grain; and two kilograms with an 
error not exceeding 0°02 of a milligram. 
The mean solar day is the natural unit of time for the human 
race, and it is universally adopted among all civilized nations. 
Our ultimate standard of time is therefore the rotation of the earth 
upon its axis, and from that rotation we determine the errors of our 
clocks and watches by astronomical observations. For many pur- 
poses it suffices to make these observations upon the sun, but when 
the utmost precision is desired it is better to make them on the 
stars. Until the close of the seventeenth century quadrants were 
employed for that purpose, and so late as 1680 Flamsteed, the first 
English astronomer royal, thought himself fortunate when he suc- 
ceeded in constructing one which enabled him to be sure of his ob- 
served times within three seconds.’ About 1690 Roemer invented 
the transit instrument, which soon superseded the quadrant, and 
still remains the best appliance for determining time Most of his 
observations were destroyed by a fire in 1728, but the few which 
have come down to us show that as early as 1706 he determined 
time with an accuracy which has not yet been very greatly sur- 
118, p. 148. 240, pp. 762 and 943. 
8 Account of the Rev. John Flamsteed. By Francis Baily. pp. 45-6. 
(London, 1835. 4to.) 
