274 PROFESSOR AIRY ON THE LATITUDE 



Had the discordance been wholly without regularity, this would have 

 given me no anxiety. But the first Aveek's observations enabled me to 

 see with certainty that one general rule could be laid down : the reading 

 for the zenith point as determined by northern stars was invariably 

 greater than that fovmd from southern stars. As the readings increase 

 while the telescope is turned towards the south, this discordance is of 

 the same kind as that which would be produced if the object end of 

 the telescope dropped by its own weight. 



After much anxious thought and many fruitless attempts to explain 

 this discordance, I was obliged to give it up entirely. The method 

 which was adopted for approximate reduction of the observations, easily 

 admitting of future correction, was the following. When in one night, 

 or in several nights which it appeared practicable to group together, 

 stars had been observed by reflexion in different parts of the meridian, 

 1 took the three means of zenith points determined by stars far north, 

 by stars far south, and by stars near the zenith, as three separate results ; 

 and then I took the mean of these three for the zenith point. For an 

 approximate co-latitude I used 37°. 47'. 6",83. 



At the beginning of March the telescope was moved about thirty 

 degrees on the circle; at the beginning of August it was again moved 

 thirty degrees, and on this occasion (as it appeared that the circle was 

 not exactly balanced) a pound of lead was attached to the eye end of 

 the telescope ; at the beginning of December it was again moved 

 about thirty degrees. It does not appear however that the fact of the 

 discordance has been affected, but its law seems to have been in some 

 degree altered. 



A discordance of the same kind exists, I believe, in every circle 

 that has been properly examined. I am informed by Mr Henderson 

 (late Cape Astronomer) that he has found it in the Cape Circle. It 

 was recognized as existing in the Greenwich Circles : and, though the 

 system of observing there, which I have described, does not allow us 

 to trace the unmixed faults of either circle, yet from the discordance 

 in the places of stars as determined by the two circles, and its variation 



