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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 a small 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 fric- 

 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 movement is surprisingly 

 easy ; and a rough graduation on the bar supplies at present 



