49 
length of the pendulum vibrating seconds. 
third position be still in excess, the truth will lie between the 
first and third positions of the slider. And thus by continually 
bisecting with the slider, the distance of the two last found 
points, the number of vibrations when the great weight is 
below , will rapidly approach the truth, being alternately in 
defect and in excess : and when the approximation is such as 
that the difference in either position of the pendulum becomes 
inconsiderable, the vibrations, when the great weight is 
below, may be taken for the truth ; and thus the number of 
vibrations in 24, hours, of a pendulum equal in length to the 
distance between the knife edges, will be known at a certain 
temperature, and at an observed height of the barometer. 
The general arrangement of the apparatus is represented by 
Plate V. 
Of the apparatus and methods employed for the measurement of the 
distance between the knife edges , and for the comparison of the 
British standard measures of the highest authority. 
The microscopes used for this purpose, were made by Mr. 
Thomas Jones, of Cockspur- street. They are both furnished 
with cross threads of spider's web, as well as with a single 
thread for the purpose of bisecting a dot if required, and are 
in other respects of a similar construction with those de- 
scribed by Sir George Shuckburgh Evelyn, in the Philoso- 
phical Transactions for 17^8, but are more powerful, and the 
micrometer is capable of far greater precision. 
The object glass of the micrometer microscope is of one 
inch focus, the distance from the object glass to the spider's 
threads 3,25 inches, the focus of the compound eye-glass 
rather less than one inch, the magnifying power 18 times. 
mdcccxviii. H 
