28 Mr. Pond's Observation 



method to measure these two arcs separately ; that is to say, 

 the zenith distance of the sun is considered as the immediate 

 object of investigation, and the remaining arc, or as it is usually 

 termed, the co-latitude of the place, (the knowledge of which is 

 presumed) is added to complete the polar distance. But in 

 whatever way we consider the subject, it is evident that the 

 sole object of the practical astronomer is to obtain by some 

 mechanical means the measure of this total arc. 



The mural circle at Greenwich has neither level nor plumb- 

 line, nor do I conceive that the least advantage could be derived 

 from the application ofeither : its principle enables the observer 

 to determine this total arc without any intermediate or zenith 

 point, hence the co-latitude, the knowledge of which is so 

 essential from the construction of other instruments, is in this 

 case rather a question of curiosity than of absolute necessity. 



I find it, however, convenient ( for reasons which I hope to 

 have an opportunity of stating more at large in a future Paper) 

 to assume an intermediate point, which I call the zenith point, 

 without being at all anxious to know whether it is really so 

 or not ; because I find myself possc ssed of the means of de- 

 termining the position of this imaginary point of departure, on 

 the instrument, to within the tenth of a second, a degree of 

 precision, which I apprehend no level or plumb-line can ever 

 be expected to equal. 



From this point of departure, I measure the distance of the 

 sun to the southward, and of the pole to the northward, and 

 the sum of these two measures is evidently the north polar 

 distance of the sun, which in every method is the ultimate 

 object to be attained. 



I have, as an example, annexed a computation of the same 



