426 Mr. Woodhouse's account of the 
the horizontality of the axis. Without attention to this latter 
circumstance the method is worth nothing ; for, if H should 
be the error in horizontality, the corresponding error in time 
would, in the latitude of Cambridge, be equal to about 28.6 
H. When we adjust according to the old plan, the colli- 
mation by means of an object near the horizon, the opera- 
tion of levelling is not required ; which in large instruments 
is rather a troublesome one ; and certainly is not, what 
M. Delambre states it to be, “ the affair of an instant/"* 
The level indicating the degree of the defect of horizon- 
tality, enables us to correct the time ;-f' and this correction is 
made on the supposition that the instrument is in the same 
state when the star is observed, as it was during its examin- 
ation by the level. It is therefore, other things being equal, 
expedient to examine by the level, as nearly as it is possible, 
at the time of observation. But this I am unable to do ; as I 
will show, by stating a circumstance rather deserving of 
attention. The tube of the telescope is braced to the axis 
(see the figure) by four tubes. The stations of the two 
* II ne faut pas commencer d’observations sans avoir rectifi^ I’horizontallte de 
I’axe, ce qui est I’afFaire d’un instant, Astron. tom. i, p. 431. 
-}• Mr. Dot.t.ovti ron<;iHer<; the value of i division of the scale of the level to be 
equal to 1". I have determined its value astronomically. Previously to a star’s 
culmination, I lowered the eastern end of the axis 10 or 12 divisions, and observed 
the star’s passages across the four first wires. I then caused the western end to be 
lowered, and observed the star’s'passages across the three remaining wires, and then 
examined the level. The following are the results : 
// 
^Cephei - - 1.014 
a Cygni - - 0.9 
J^Draconis - - 1.005 
a Cephei - - .855 
Polaris - - 0.9516 Meano".945i. 
