BY G. R. SMALLEY, ESQ. :; 1 1 



tude, and X ursa? minoris of the fifth Magnitude neither of 

 whose angular distances from the Pole much exceed a degree ; 

 and in the Southern Hemisphere we have cr Octantis, a star of 

 the sixth Magnitude, whose angular distance from the Pole is 

 about | of a degree; and by the application of Observations 

 made upon one of these stars, in a manner familiar to the 

 astronomer, we are enabled to determine the azimuthal error of 

 the meridian instrument with as much essential accuracy as if we 

 strictly had a Pole star to guide us. 



This error being now determined, it is easy to erect a fixed 

 azimuth mark so far distant from the observatory that any small 

 accidental change in its position be imperceptible. Such 

 a mark is of great importance to enable the astronomer to 

 determine the position of his instrument at times where he is 

 prevented from observing his azimuth stars. It is usual to have 

 this mark constructed of stone or masonry with a fine distinct 

 mark engraved upon a plate embedded into the upper part. 

 Near St. Leonard's Church, on the North Shore, there is a tree 

 iu the last stage of rottenness, having attached to one of its 

 branches an iron ring with two cross wires this constitutes the 

 azimuth mark of the Sydney Observatory ! * 



Let me now proceed to consider the circumstances of the 

 Royal Observatory at Greenwich. 



The constitution of Greenwich Hill, is, I believe, gravel with 

 h substratum of clay. The Observatory itself is about 150 feet 

 the mean level of the Thames, and the direct distance from 

 the river is about half a miie. It stands, speaking roughly, 

 about lOu feet from the edge of the north side of the hill, where 

 it slopes rapidly down towards the Thames ; at about the same 

 distance also, there is a considerable slope towards the west ; and 

 at the distance of a mile or thereabouts on the south and south- 

 west lie the vallies of Blackheath and Lewisham : towards the 

 east there is little or no fall for about three miles. I speak 

 roughly from memory, but I do not think I am far wrong in my 

 descriptions of the locality the height of the Observatory I have 

 given accurately. 



* Since this paper was written, I have succeeded in re-placing the tree 

 by a substantial stone column. G. R. S. 



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