ON DTTIDING ASTRONOMICAL INSTRUMENTS. ] J | 



of reading' off are not errours of division, let tliem be dis- 

 charged from both, and the erroiirs will then stand, for the 

 quadrant 2", and for the circle 3-"85. As the radius of the 

 former, however, is fonr times greHler tiiavi t!^at of the lat- 

 ter, it will appear, by this mode of trial, that the eqnato-* 

 rial is rather inote than twice as accurately divided as the 

 ouadrant. In doing justice to Bird in this instance, I have 

 only done as I would be done by : for, should any future 

 writer set me back a century on the chronological scale of 

 prof.'Tessive improvement, I hope some one will be found to 

 restore me to my proper niche. I now subjoin a restate- 

 ment of the greatest errour of each of the instruments tliat 

 are brought into comparison by Sir George, after having re- 

 duced, them all by one rule; viz. allowing each of the two 

 points vvhich bound the njust erroneous extent to divide the 

 apparent errour equally between them. They are expressed 

 in parts of an inch, and follow each other in the order of 

 their accuracy. 



Sir George Shuckburg's 5 feet standard • • • • •00016.5 ^rro 

 General Rov's scale of 42 inches -000240 



11 rs oi (lit 

 f.'rent sun- 



tiards. 

 Sir George's equatorial, 2 feet radius -000273 



The Greenwich quadrant, 8 feet radius • • . • '000465 



Mr. Aubert's standard, 5 feet long '000700 



*The Royal Society's standard, 92 inches long, 'OOO.SS 



For the jiastness of the above statement I consider my 

 name as pledged ; requesting the permission to say, that if 

 on the result of each respective esauiination, as here pre- 

 sented, there could have been more than one opinion, it 

 would not have appeared here. I am farther prompted to 

 add, that the above comparative view presents one circum- 

 stance to our notice, which cannot do less than gratify 

 every individual, who is at all conversant in these matters ; 

 I mean, the high rank which General Roy's scale takes in 

 the list; that scale having been made the agent in measur- 

 ing the base line of our national trigonometrical survcv. 



To return, finally, to the dividing of circles; 1 must Ca?e must be 



state, as matter of precaution, that great care should be *'^'^'^"' ^" '''"^ 



^ the Circle of a 



• This is the same which Mr. Bird used in dividing his eight feet nmral' 

 quadrants, and was presented to the Royal Society by Bird's exetutors. 

 ■ " taken 



