length of the pendulum vibrating seconds. 5 1 
formed by them appeared to be accurately bisected by the 
line. The division of the micrometer was then noted, and 
this was repeated several times with scarcely a sensible dif- 
ference in the result; and thus I assured myself that no error 
worthy of remark was to be apprehended from imperfection 
of vision. 
The next step was to determine the value of one division 
of the micrometer head. By the kind interest of Sir Joseph 
Banks, I was favoured with the use of the standard scale 
which belonged to the late Sir George Shuckburgh Evelyn, 
and which is described in the Philosophical Transactions for 
1798. This scale, the work of Mr. Troughton, is second 
to none in the kingdom in point of accuracy of division, and 
is too well known to render any further remark necessary. 
The microscope being carefully adjusted for parallax, one 
inch, from the 39th to the 40th, was measured by successive 
tenths, and the mean taken as the value of one tenth of an 
inch. The measurement of the same inch was repeated ten 
times at different periods, the microscopes being previously 
adjusted anew each time for parallax. The mean results of 
such measurements are as follows. 
Divisions of the micrometer to — of an inch. 
2335,°° ' 
2S33 >75 
2337,55 
2337 >32 
2 334>5° 
2336,9° 
2 335,75 
2338,30 
2335>85 
2337> 8 5 
Mean 2336,277 
