VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. l63 



recollect the size of the instrument as well as its extensive use, since I can not 

 only follow any object for near a quarter of an hour, without disturbing the situ- 

 ation of the apparatus, but can at pleasure, in a few minutes, turn it to any 

 part of the heavens, and view a celestial object wherever it may chance to be 

 situated, even the zenith not excepted. 



From this account it will be understood, that the places of a few of the nebulae 

 and clusters of stars, determined before the 13th of December, 1783, may be 

 faulty in right ascension as far as I'^of time, and in polar distance to 8 or JO'of 

 space. Afterwards the errors will be found to become gradually less considerable 

 till the latter end of the year 1784, when I suppose they will seldom exceed half 

 that quantity. From that period to Sept. 24, 1785, they will diminish, and 

 probably not often amount to so much as 3 or a! in polar distance, and 10 or 12* 

 in right ascension. And now I flatter myself that all places, determined since 

 the last mentioned time, will generally be true to a very small quantity; such as 

 4 or 6* in right ascension, and 1-^or 2' in polar distance, and often much nearer. 



Some of the nebulae in that part of the heavens which, in a former paper, I 

 have called the stratum of Coma Berenices, are indeed so crouded, that there 

 was no possibility of taking them all in the centre of the field of view, and a 

 somewhat less degree of accuracy may therefore be expected; but having used 

 myself by very frequent estimations of the parts of the field of view to judge 

 of their value in time as well as in space, I corrected this defect at the moment 

 of observation by afilxing to the transits of these excentric nebulae, such proper 

 marks of plus or minus in right ascension and polar distance, as I judged would 

 bring them to a central observation. A similar method, well known to good as- 

 tronomers in estimating their lOths of seconds by the proportional space over 

 which the stars move in their meridian passage, makes it unnecessary to expa- 

 tiate on the degree of accuracy that long practice enables us herein to obtain. 



If however I had been willing to delay giving this catalogue till, by a repeated 

 review of the heavens, the places had been more accurately determined, the 

 work would undoubtedly have been more perfect; but whoever considers that it 

 requires years to go through such observations will perhaps think with me, that 

 it is the best way to give them in their present state, if it were but to announce 

 the existence of such objects by way of inducing other astronomers also to look 

 out for them. Another motive for not delaying this communication, is to show 

 that my late endeavours to delineate the construction of the heavens have been 

 guided by a careful inspection of them; and probably a catalogue which points 

 out no less than 1000 instances of such systems as those are into which I have 

 shown the heavens to be divided, will considerably support what has been said on 

 this subject in my two last papers. 



When the diurnal motion of the earth was first maintained, it could not but 



Y 2 •■ /' ■' 



