430 Introduction to the Sixth Section 



tected as far as possible against the effect of a general sinking 

 of the ground, as the firmness and solidity of the connexion 

 of all parts render any rotatory motion very improbable ; and 

 lastly, the moving of the observer has no effect on them, as 

 the floor does not rest on the stone slab, but on the surround- 

 ing wall, and as between the floor and both the slab and the 

 pillars there is an empty space. 



The instrument being constructed for reversing, which is 

 indeed one of its most beautiful advantages, care was to be 

 taken that the reversing itself should produce no change of the 

 pillars. The end of the axis which carries the two circles is 

 more heavy than the opposite one, and therefore requires 

 heavier counter-weights, by which the levers exert on one side 

 a greater pressure by 104- pounds than on the other. The 

 axis being levelled and the instrument reversed, the pillar 

 which before sustained the greater pressure, now suffers the 

 less, and vice versa,; and it cannot be doubted that a difference 

 of 208 pounds will produce a sensible change in the horizon- 

 tality of the axis. In order to obviate the error proceeding 

 from this circumstance, there were fixed in the wall sur- 

 rounding the base, two strong iron bars, passing along the 

 pillars from south to north without touching either the stone 

 slab or the floor. From these bars proceeds vertically along 

 the middle of each pillar a weak iron rod, which by means of 

 a quadrangular frame incloses the Y's of the instrument. On 

 the upper end of the latter rod on one end a lever may be 

 made to act, which by a weight applied on the opposite end, 

 exerts a pressure of 104 pounds. This lever is always applied 

 to the side opposite to that where the circle is attached, and 

 thus most completely removes the cause of the error. With- 

 out this apparatus there would be in the two positions of the 

 axis a difference of 1"*3. 



However perfect the meridian circle of lieichenbach may 

 be, I did not conceive that I ought to give up the opinion, on 

 which I had hitherto always acted, — viz. that all instruments 

 ought to be subjected to an accurate and rigorous examination 

 after they are placed in the observatories. In regard to this 

 instrument, a two-fold investigation is required : first, on the 

 curve described on the celestial sphere by the line of colli- 

 mation; and next, on the points of this curve answering to va- 

 rious distances from the pole and zenith. Only the first of 

 these investigations has hitherto been accomplished. I shall 

 therefore now explain the examination of the instrument, 

 when used as a transit ; reserving for the next Section the re- 

 port on its use for measuring zenith distances. 



1. Dc- 



