420 PHILOSOPHICAL TRANSACTIONS. [aNNO 1747-8. 



being too small to admit of its being turned to the opposite side of the wall, 

 where it now hangs ; I cannot, by actual observations of the circumpolar stars, 

 settle those necessary points; and therefore have endeavoured to do it, by com- 

 paring my own with his lordship's observations : and until this defect in the ap- 

 paratus belonging to the royal observatory be removed, we must be indebted to 

 his lordship for the knowledge of its true situation. 



A mind intent on the pursuit of any kind of knowledge, will always be agree- 

 ably entertained, with what can supply the most proper means of attaining it. 

 Such, to the practical astronomer, are exact and well-contrived instruments ; and 

 I reflect with pleasure on the opportunities I have enjoyed, of cultivating an ac- 

 quaintance and friendship with the person, that of all others has most contri- 

 buted to their improvement : for I am sensible that if my own endeavours have, 

 in any respect, been effectual to the advancement of astronomy, it has princi- 

 pally been owing to the advice and assistance given me by our worthy member 

 Mr. Greorge Graham ; whose great skill and judgment in mechanics, joined with 

 a complete and practical knowledge of the uses of astronomical instruments, en- 

 able him to contrive and execute them in the most perfect manner. 



The gentlemen of the Royal Academy of Sciences, to whom we are so highly 

 obliged for their exact admeasurement of the quantity of a degree under the arctic 

 circle, have already given the world very convincing proofs of Mr. G.'s care and abi- 

 lities in those respects ; and the particular delineation which they have lately 

 published, of the several parts of the sector, which he made for them, has now 

 rendered it needless to enter on any minute description of mine at Wansted ; 

 both being constructed on the same principles, and differing in their component 

 parts chiefly on account of the different purposes for which they were intended. 



As mine was originally designed to take only the differences of the zenith dis- 

 tances of stars, in the various seasons of the year, without any view of discover- 

 ing their true places ; I had no occasion to know exactly what point on the limb 

 corresponded to the true zenith : and therefore no provision was made in my 

 sector for the changing of its situation for that purpose. Neither was it neces- 

 sary that the divisions or points on the arc should be set off, with the utmost 

 accuracy, equidistant from each other ; because, when I observe any particular 

 star, the same spot or point being first bisected by the plumb-line, and then the 

 screw of the micrometer turned till the star appears on the middle of the wire, 

 fixed in the common focus of the glasses of the telescope ? I can thereby collect 

 how far the star is from that given point at the time of observation : and after- 

 wards, by comparing together the several observations that are made of it, I am 

 able to discover what apparent change has happened. The quantity of the visible 

 alteration in the position of the stars, being expressed by revolutions and parts of 

 a revolution of the screw of the micrometer; I endeavoured to determine, wit^ 



