Memoir on the Repeating Reflecting Circle. By Major- 

 General Sir Thomas Brisbane, F. R. S. E. Corresponding 

 Member of the Academy of Sciences. 



(Read Feb. 15. 1819.^ 



XXaving had frequent opportunities, during my residence in 

 France, of seeing a great many Repeating Reflecting Cir- 

 cles, several of which I have observed with, and having found 

 much consistency in the results deduced from observations 

 made with them, it occurred to me, that if I could engage Mr 

 Troughton to execute one for me, it would be a most perfect 

 instrument. This he accordingly did; and I trust that the 

 observations made with it, which I am about to communicate, 

 will perfectly justify the opinion I had formed of its precision. 

 The instrument is only of about six inches radius, divided in 

 gold to 20". I have always been in the habit of choosing for 

 latitudes 30 repetitions, when the weather would permit, divi- 

 ded into three series of 10 each, and reading the angle or arc 

 run through during the series, which completely destroys every 

 imperfection of the division, or reading. In order to deter- 

 mine the time, I have always found six repetitions quite suffi- 

 cient to give it as correct as equal altitudes. Of these I also 

 generally observe three series, in order to take the mean. 

 These observations I have repeatedly verified by equal alti- 

 voL. IX. p. I. N tudes. 



