Mr. Pond on a derangement in the mural circle. 87 
Greenwich Mural Circle, are aware, that though the telescope 
may be applied to every part of the circle, yet, when fixed 
for observation, the principle of the instrument requires that 
the tube, especially at its extremities, should 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 appears that these braces have, in progress of time, 
become insecure, owing to the screws which fastened them 
having 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, 
remains to be determined by future observations. 
The cause of this error being thus ascertained, Mr. 
Troughton has applied additional braces to connect the 
telescope with the circle, sufficiently strong, I should con- 
ceive, to prevent the possibility of such an accident for the 
future. 
This alteration has already produced such an improve- 
ment in the observations, as prove sufficiently that the source 
of error has not been mistaken. Of the published observa- 
tions, 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 after- 
wards increased. 
