• 15^ Captain Kater's description of 
wire, and the mean of seldom less than three readings re- 
gistered. 
The stability of the micrometer was severely proved by 
pressing upon the end of the telescope, and it was found that 
upon removing the finger, the micrometer wire always re- 
turned precisely to the same situation. The value of each 
division of the micrometer head is six-tenths of a second. 
In the following tables, the first column contains the read- 
ings of the micrometer when the angle of the cross wires was 
bisected. The second column contains the difference between 
every two consecutive observations, and indicates the derange- 
ment of the position of the float from its having been moved 
between such observations. The third column contains the 
error in seconds, which would affect the determination of 
the horizontal point in consequence of the derangement of 
the float, and is equal to the half of that derangement. 
Wooden Float. 
ist Set. 
Divisions 
of 
Microm. 
Difference. 
Error in Sec®, 
affecting the 
Horiz. point. 
Dec. 5 
✓ 
Previously to moving - - - 
O raised - - - 
E raised - - - 
S raised - - 
S s raised - - - 
O raised and carried 
E raised and carried 
S raised and carried 
S s raised and carried 
85,4 
82,6 
80,9 
79»3 
79 »S 
7°>9 
7*>9 
70.3 
+ 
— 2,8 
— *>7 
— 1,6 
+ 0,2 
— 8,6 
+ 
— 1,6 
4* 0,45 
— 0,84 
— 0,51 
— 0,48 
+ 0,06 
— 2,58 
+ 0,30 
— 0,48 
The image of the cross wires not being perfectly distinct, 
I limited the aperture to three-quarters of an inch, and thus 
