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death he ordered a complete circle, of 6 feet diameter, to he 
made, with a telescope, of 4-inch aperture, fixed upon it, gra- 
duated around its periphery. This circle turned upon a pivot 
carried through a wall to which were fixed six micrometers for 
reading the circle divisions. Maskelyne did not live to use 
this “ Mural circle,” but Pond had the advantage of it ; and he 
subsequently had another one made like it, using the two 
together. Pond also had a new Transit made by Troughton, of 
5-inch aperture, which was a masterpiece ; and with these means 
he pushed observing accuracy to a point that had not before 
been aimed at, and has even now scarcely been surpassed. 
The essential parts of all these instruments (except Flam- 
steed’s) are shown in the accompanying sketch, upon a scale of 
Fig. l. 
1. Halley’s Transit (1721-1749). 4. Mural Quadrant (1725-1812). 
2. Bradley’s Transit (1750-1816). 5. Troughton’s Transit (1816-1850.) 
3. Bradley’s Zenith Sector. 6. Mural Circle (1812-1850). 
about J inch to the foot : the cut also shows the form of 
Bradley’s famous Zenith Sector, with which he consummated his 
immortal aberration discovery. 
The present Astronomer Eoyal, Sir George Airy, used Pond’s 
instruments as he found them until about the year 1850, when 
finding them too small in their object-glasses for existing and 
future wants — especially for the observations of the newly-dis- 
* The woodcuts illustrating this article have been kindly lent by Messrs, 
Bell and Daldy, and Messrs. Bradbury and Evans. 
