OF CAMBRIDGE OBSERVATORY. 273 



calculation for the zenith point then relies on our perfect acquaintance 

 with the variations of refraction and other corrections from one night 

 to another ; and thus a cause of inaccuracy is introduced, which does 

 not exist in the other method. In the Cambridge Observatory a different 

 method is regularly employed (for the idea of which I am indebted 

 to a suggestion of Mr Sheepshanks). When a star is to be observed 

 by reflexion, the circle is set approximately for the reflected observation, 

 and the six microscopes are read; when the star has entered the field, 

 and before it has reached the center, it is bisected by the micrometer 

 wire, (which in fact measures its distance from the fixed wire, and thus 

 gives a correction to be applied to the mean of the six microscopes,) 

 and then there is ample time to allow the circle to be turned to the 

 position in which the star can be observed directly, shortly after it 

 has passed the center of the field. Thus a direct and reflected observa- 

 tion are obtained at the same transit. This method is, in my opinion, 

 much preferable to the second that I have mentioned, and in some 

 respects superior to the first. 



Either of the methods which applies to one circle enables us, as 

 will shortly be seen, to examine severely into the consistency of the 

 results obtained in different positions of the circle ; and this must be 

 considered as a most valuable property of this method of determining 

 the zenith point, and one which places it far above the use of a collimator 

 or any similar instrument. 



I had hoped, on commencing observations with the Mural Circle 

 at the beginning of the year 1833, to be able in a very short time to 

 obtain a very approximate latitude. I proposed to observe some stars 

 every night in the manner above described, as well as circumpolar stars 

 (which might or might not be observed in the mercury): by the former 

 I should obtain a very good zenith point; and then each observation 

 of the latter, above and below the pole, would give me a value of the 

 co-latitude. 



But after a few nights' observations, I found that the reading for 

 the zenith point, as determined by different stars, was not the same. 



