LATELY ERECTED AT THE ROYAL OBSERVATORY. 151 



This method should be employed when the object is to judge of the consistency of 

 observations, without any reference to the astronomical result. 



Table III. shows the manner in which the difference in zenith distance between 

 the two stars is obtained by means of the circles ; a quantity, as I have shown, of 

 the highest importance in the investigation. 



This quantity, having been determined by the microscopes of the respective circles, 

 might be erroneous if the runs of the microscopes were not exact, although the error 

 here must be very small, twelve microscopes being constantly used. But as they 

 have lately been taken down, examined, and replaced, without any sensible alteration, 

 it may be presumed that the error from this source is sufficiently corrected. 



Table IV. This Table contains in a very compressed form the result of an immense 

 number of observations of y Draconis during a period of eighty years ; and it will be 

 seen that if from M. Bessel's formula*, deduced from the first sixty years of these 

 observations, we attempt to predict or assign the place of the star for the present 

 time or twenty years in advance, the star will be found 1"75 south of its computed 

 place. 



* By this formula the zenith distance of the star north for 1800, + t = 2' 26"-669 - t . 0"-71394 + t"- . 

 0"-001011. Where t is the number of years before or after 1800, if before, the sign of t is minus. 



