88 Mr. Pond on a derangement in the mural circle. 
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 
altitude 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 
increased; and this ultimately led to a discovery of the cause. 
My present object in this letter, is simply to state these 
circumstances to the Society. I shall defer a more detailed 
account of 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 degree of certainty, at what period the derange- 
ment took place, and what corrections are required to be 
applied, till the instrument was restored to its perfect state. 
I have the honor to be, 
My Dear Sir, 
with the highest regard, 
your most obedient humble Servant, 
JOHN POND, 
Royal Observatory s 
Nov. 21, 1821. 
