356 Derangement in the Greenioick Mural Circle. 



amount of tliis error, and the correction which should be ap- 

 pHed to the observations made within the two last years. At 

 present it will be sufficient to mention, in as few words as pos- 

 sible, the cause of this error. 



Those who are acquainted with the construction of the 

 Greenwich Mural Circle, are aware, that though tlie telescope 

 may be applied to every part of the circle, yet, when fixed lor 

 observation, the principle of the instrument requires that the 

 tube, especially at its extremities, k! juld be so firmly fixed 

 to the circle as to form one piece with it: to accomplish this, 

 connecting braces are attached at each end of the telescope. 

 It now a])pears that these braces have, in progres of time, 

 become insecure, owing to the screws which fastened them 

 liaving given way. The effect of this will be, to permit the 

 ends to bend from the centre instead of retaining, as they 

 ought to do, an invariable position with respect to the circle. 

 Under these circumstances, when the telescope is directed to 

 the zenith, the position may be considered as free from error; 

 but when the instrument is moved either towards the north 

 or south horizon, should either extremity bend more than 

 the other, an error will take place, and will increase from the 

 zenith towards the horizon, but in what exact proportion, re- 

 mains to be determined by future observations. 



The cause of this error being thus ascertained, Mr. Trough- 

 ton has applied additional braces to connect the telescope with 

 the circle, sufficiently strong, I should conceive, to prevent 

 the possibility of such an accident for the future. 



This alteration has already produced such an improvement 

 in the observations, as prove sufficiently tliat the source of 

 error has not been mistaken. Of the jmblished Observations, 

 only those made in the three last months of the year 1819 are 

 affected by this error, and that in so very small a degree, as 

 must have entirely escaped notice, had it not afterwards in- 

 creased. 



During the year 1820 the error increased; but did not, I 

 believe, in the distance from the pole to the equator amount 

 to two seconds ; at altitudes lower than Sirius, and at the al- 

 titude of the sun at the winter solstice, the error may have 

 been greater than two seconds, but did not exceed four. 



But after the- month of February 1821, the error rapidly in- 

 creased ; and this ultimately led to a discovery of the cause. 



My present object in this letter, is simply to state these cir- 

 cumstances to the Society. I shall deter a moi'e detailed ac- 

 coimt ol" the nature of this derangement, and of its effects, till 

 I shall be enabled, by a sufficient number of observations made 

 with the instrument in its improved state, to ascertain, with 



some 



