OF CAMBRIDGE OBSERVATORY. 275 



in different points of the meridian, I am inclined to think that the 

 defect in one circle is different from that in the other. 



In vain have I endeavoured to discover the cause of this discordance. 

 I once thought that it might be owing to the circumstance, that for 

 the reflection-observation the circle is at rest for some minutes after 

 the microscopes are read, and possibly it might (though clamped) have 

 changed its position. A series of observations expressly made, showed, 

 however, that there was no sensible change either in a few minutes 

 or in many hours. I thought that the surface of the mercury might 

 be sensibly curved, and that from a habit of observing in one part of 

 the trough, an error might be produced. A set of experiments proved, 

 however, that there was not the least sensible difference in the results 

 found from observing at one or the other end of the trough. A flexure 

 of the wire in the field of view would not explain it, as the discordance 

 which that would produce is of the opposite kind. There appeared 

 to be no reason for supposing an error in the determination of the 

 coincidence of the micrometer wire with the fixed wire, in the value 

 of the micrometer screw, or in the observation with the micrometer 

 wire. The object glass, repeatedly examined by myself and once by 

 Mr Simms, did not appear to be loose in its cell. I am driven at last 

 to the supposition that the circle sensibly changes its figure ; but I 

 have no proof of this, nor do I see distinctly how it should produce 

 the discordance in question. Three sets of readings of every 10° under 

 all the microscopes, have not assisted me to discover such change. 

 My a priori opinion is, that a change in figure is hardly possible. The 

 telescope, it must be remembered, is attached at its ends to the limb 

 of the circle : the limb is in one piece (cast in several pieces and burnt 

 together) ; and the whole arrangement of parts seems admirably adapted 

 to prevent any change. If I had to fix on an astronomical instrument 

 which appeared less likely to change than any other, I should certainly 

 choose the Mural Circle. 



To discover experimentally the law of discordance, I proceeded 

 as follows. The observations being reduced, and those of each star 

 being digested under the heads of D, R, SP. D., and SP. R., I 



