150 



MR. POND ON THE NEW ZENITH TELESCOPE 



Table IV. 

 Fundamental determinations of the Zenith Distances of y Draconis. 



The above results (column 3rd) are those that have been obtained with the greatest 

 care during their respective periods ; and having been deduced from observations with 

 the zenith sector, they are quite independent of the latitude of the Observatory. 



M. Bessel's formula is deduced from the observations for the first sixty years, 

 and therefore agree very well ; but when we attempt to predict from the observations 

 of these sixty years the place of the stars for twenty years to come, we find a differ- 

 ence of l"-74 between the predicted and observed zenith distance, the observed place 

 being this quantity south of it. 



Explanation of the foregoing Tables. 



Table I. contains the results of 60 observations of the small star Bode 170 Draco- 

 nis, made with Jones's circle, and is intended to show what degree of accuracy may 

 be obtained by extreme care. The mean difference 0"-357, column 3, between the 

 mean of the whole and each result, (and which is nearly the probable error of a 

 single observation from this series,) demonstrates with what care they have been 

 made. The same may be said with respect to the mean difference of column 4, 

 namely, 0""155, which is similarly obtained from the mean of the whole and the 

 mean of each ten (a quantity which represents nearly the probable error of the mean 

 of ten observations). However, it may be remarked, that the exact coincidence ex- 

 hibited throughout this series does not prove the truth of the final north polar 

 distance of the star here assigned, since some omissions or errors in the process of 

 reduction would affect it. That no instrumental error exists is demonstrated by the 

 identity of the result with that obtained with the new instrument. 



Table II. contains the same observations arranged in a different manner. 



This is the arrangement I have advantageously followed in investigating the differ- 

 ence of parallax ; the object being to distinguish the effect arising from accidental 

 error of observation from that which is due to any permanent astronomical cause. 



