SECOND REPORT - 1832 
132 
A smaller instrument of the same construction, well known as 
the Westbury Circle, was in the hands of Mr. Pond : and by 
the use of this, it may be asserted, the errors of the large 
Greenwich quadrants were first completely established. In 
1809 Mr. Troughton published in the Phil. Trans, an account 
of his method of dividing circles ; and this may he considered 
as the greatest improvement ever made in the art of instrument¬ 
making. The general principle is to make a number of tem¬ 
porary points very near the places of many of the graduations, 
to compare by microscopes the distances between every pair, 
and when the errors are found numerically, to set off by a simple 
apparatus the permanent points at the proper distances from 
the temporary points*. In-1812 the first mural circle (by 
Troughton) was erected at Greenwich ; and this is an important 
epoch in the history of Astronomy. I conceive that no instru¬ 
ment but the reversible circle can compete with Troughton’s 
mural circle ; and between these I cannot presume to decide. 
It must be observed that, as the mural circle was first intended 
to be used, the objects of these instruments were somewhat 
different. The reversible circle could be turned round its 
vertical axis in a few minutes, and the deviation of the axis from 
perfect vertically could be ascertained by the plumb-line; and 
the body under observation being observed in both positions 
of the circle, its zenith-distance was directly found (a small 
correction being necessary to obtain its meridian-zenith-distance, 
as both observations could not be made on the meridian. But 
the mural circle, like the mural quadrants, had no reference to 
the zenith ; it could give only the polar distance of heavenly 
bodies, the position of the instrument corresponding to an ob¬ 
servation of the celestial pole being found by observing circum¬ 
polar stars above and below the pole. To remedy this want 
(sometimes felt as an inconvenience,) Mr.Pond introduced (about 
1821,) the system of observing sometimes the image of the 
heavenly body seen by reflection from the surface of mercury. 
In 1825 another instrument of exactly the same kind was 
erected; and now the system may be said to have reached its 
perfection. Whenever the weather permits, the same object 
is observed directly with one circle, and by reflexion with the 
other. The determination of zenith-distances from the combi¬ 
nation of these observations, though laborious, must I think be 
unrivalled in point of accuracy.—Several circles on Troughton’s 
plan have been sent to continental observatories. 
* Several methods, slightly different, have been founded on this of Trough- 
ton’s. I do not know what method the continental artists employ. 
