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that their action is a maximum when the tube is level, and 
nothing when it is vertical; but between these positions it 
decreases more slowly than the downward tendency of the 
tube. To correct this, the tube is connected with loaded 
levers (placed to its south), by chains of such lengths that 
one of them is not raised till it is at 40° altitude, and the other 
at 80°; the latter being necessary for the return of it after it 
has passed the zenith. The slow motion in declination was 
not yet applied, but the ordinary one was quite convenient, 
except for the difficulty of giving orders to the men, who were 
sometimes seventy or eighty yards from the observer. A two- 
feet circle, with a fine level and a pair of verniers, will also be 
attached to the tube to give the declination ; its place was then 
supplied by asmall protractor, five inches diameter, over which 
was a strong screen to protect the assistant who attended it 
from any such casualty as the fall of an eye-piece. 
The eastern pier bears what may be called the meridian of 
the instrument: it is a strong semicircle of cast iron, about 
eighty-five feet diameter, and composed of several pieces accu- 
rately planed. Each of these is bolted to the pier and sepa- 
rately adjustable to a meridian line formed by straining a fine 
wire over notches in two cast iron chairs firmly secured at the 
north and south of the masonry. Sir James South took charge 
of this delicate operation, and performed it with such precision 
that when a transit instrument was adjusted by this line, it 
gave the passage of Polaris to a small fraction of a second. 
The telescope is compelled to move in the meridian, being 
connected with this circle by a strong bar provided with frie- 
tion rollers, that it may traverse it easily; and thus it can be 
used as a transit instrument with considerable precision. But 
this bar is racked, and attached to the tube by wheelwork, so 
that a handle near the eye-piece enables the observer to move 
it on either side of the meridian, and thus examine it before its 
passage, or follow its motion. The movementis surprisingly 
easy ; and a rough graduation on the bar supplies at present 
