86 Differences of Meridian at Madras, [No. 97. 



the circumference ; and what is of great consequence, the obser- 

 ved objects have the same, or very nearly the same, degree 

 of light, so that the eye has not to change its focus and 

 condition; besides, if the reading of the three indexes take 

 up more time, it is a very little more than the reading of 

 one, it is amply repaid by the time gained, in not being 

 obliged to take the index error. 



In Mr. Troughton^s paper, he, in his usual clear manner, 

 explained the adjustments and mode of using his circle. I 

 give one more extract from it, to shew the opinion of him, 

 allowed to be the best artist in Europe, of its value ; he says — 



" The greatest error, to which dividing by a good engine 

 " is liable, may be taken at about twenty seconds ; the six 

 " readings required in a double observation on different parts 

 c< of the circumference, will probably reduce that error to 

 " within five seconds, where the reflecting glasses and teles- 

 " copes are good, and power considerable (about twelve) a 

 eg mean of contacts will come out within this quantity, and 

 ct where every other source of error is corrected by the prin- 

 " ciples of the instrument, we are of opinion, that a series of 

 " lunar observations will give the longitude on land, nearly, 

 " if not quite, as accurately, as can be obtained from an occul- 

 u tation of a star, by the moon, when observed with a powerful 

 " telescope." 



It is well known that Mr. Troughton made more and 

 better sextants than any other artist, and of course derived much 

 profit by their sale, yet such was his disinterested desire that 

 his circle should come into general use, that he made the 

 price only one guinea more than that of his best sextants, 

 though the real difference of cost in material and workmanship 

 is considerable. On the same terms, and with the same ex- 

 cellence of execution, Troughton's reflecting circles are now 

 supplied and constructed by his worthy successor, Mr. William 

 Simms, F.R.S., an artist whom Mr. Troughton selected as best 

 worthy to sustain his great reputation. 



The chief reason why the circle has not come into more 

 general use at sea, is its greater weight than the sextant, and 

 the partiality men feel for instruments they have been used to 3 



