the parallax of a. Lynx. 
Considering the manner in which the microscopes are 
placed on the two instruments, theory must be in favour of 
the superior steadiness of those of the Greenwich instrument, 
but experience teaches us quite otherwise. 
The last column of part ( 2 ) of this Table will, I conceive, 
appear highly worthy of attention. It is half the sum of the 
columns WM and EM, and therefore is the difference for each 
day between the zenith distances determined by the bottom 
microscope, and by the mean of the side microscopes. 
It shows that the different temperatures of winter and 
summer do not operate a change in the figure of the circle, 
at least in an arc of 14 0 , and so in this respect can have no 
reference to the parallax of u Lyras. 
The same column also shows the great exactness of the 
readings, the very few instances of discordances in the co- 
lumn may be either attributed to slight temporary derange- 
ments in the microscopes, as is evidently shown, September 1 a, 
in column WR, or to the small errors of reading lying so as 
to appear with an accumulated effect. 
If B, L, R represent the readings of the bottom, left, and 
right hand microscopes, when the face of the circle is west, 
and B' L ' R' the same when the face is east ; the quantity 
in the last column of this Table, part (2) = 
i(B 
L + R 
J3' hl±JL' j __ B + B' __ i | L + 1/ R + R‘ 
the mean of the differences between the zenith distance by 
the bottom and each of the side microscopes respectively. 
It will seldom happen for other stars that the number in 
the last column will be so very small. But the equality of 
the numbers is the circumstance to be reckoned on here, not 
their magnitude. 
mdcccxxiv. 
3 S 
