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X. A Letter from John Pond, Esq. Astronomer Royal , to Sir 
Humphry Davy, Bart. President of the Royal Society , rela- 
tive to a derangement in the Mural Circle at the Royal 
Observatory. 
Read November 22, 1821. 
My Dear Sir, 
The interest which the Royal Society has always taken in 
every thing relating to this Observatory, and to which maybe 
attributed its present prosperous condition, will, I trust, render 
unnecessary any apology for this communication. 
I wish to make known to astronomers, as soon as possible, 
a derangement that has for some time past existed in the 
mural circle, and of which I have not, till lately, been able 
to ascertain the cause with certainty. 
This derangement began, I believe, about the autumn of the 
year 1819; the position of the telescope was then changed; 
and from that time the error has been gradually increasing 
till last summer, when the cause was distinctly ascertained, 
and the proper remedy applied. 
In the Preface to the Greenwich Observations for the year 
1820, now printing, I shall have an opportunity of stating the 
amount of this error, and the correction which should be 
applied to the observations made within the two last years. 
At present it will be sufficient to mention, in as few words as 
possible, the cause of this error. 
Those who are acquainted with the construction of the 
