92 Sir George Shuckburgh's Account 
error exceeding 3 or 4" need be apprehended, if proper care is 
taken. 
The swinging level l, m, fixed to the tube of the tele- 
scope, for the purpose of taking declinations, is unground, and 
of very superior sensibility, as it ought, having been selected 
expressly from a great many yards of glass tube, in order to 
get one with a proper flexure ; and such has been the success of 
this care, that a change of inclination of this level of one single 
second, moves the bubble nearly ~ of an inch, certainly more 
than ~* This, I am inclined to believe, is the greatest sen- 
sibility that has ever yet been attained by a level : it is at the 
same time somewhat inconvenient, since it sometimes will 
require two or three minutes time to settle to its true point, 
it moves so slow. An error, however, of more than a second 
need seldom be apprehended, which I conceive is as much as 
could have been expected from a plumb line of a length equal 
to the diameter of the circle. The . bubble of this level is 
about 7 inches long, but this varies with the temperature ; and 
I have experienced that with about 28° of heat the bubble 
contracts one inch in length ; this makes it very necessary to 
be attentive to the index at each end of the bubble. The 
parts of this level may be seen at figure 2, Tab. XIV. 
A,B, is one side of the tube of the telescope, whereon are 
firmly fixed two upright supporters, or cocks, C, D, and E, F ; 
* If the motion of the bubble be taken to be even 0,2 inch, it will be found that the 
radius of curvature is more than 1 100 yards ; and, supposing the length of the level 
— 12 inches, it will only comprise, from one end to the other, an arch of 1', and the 
versed sine of 30", in this case, would express the depressure of the tube at its extre- 
mities below the centre of the level about half T -<™ of an inch. 
The curious ground level, for adjusting the axis of the transit instrument at the ob- 
servatory at Greenwich, moves, as I have been informed, about 3-5 inch for 1". 
