374 Mr. de Mendoza Rios on an improved Reflecting Circle. 
as well as in Borda’s, there is a great objection to any attempt 
for that purpose; because, as the horizon glass moves round 
with the index, its perpendicular position is deranged by the 
inequalities of the plane of the limb ; but, in my construction 
for multiplying with the horizon glass fixed, that inconvenience 
is removed ; and the method of ascertaining the identical posi- 
tion of the glasses may be employed in practice, with advantage, 
it being done when the index is at the same point of the frame. 
By suggesting this idea, I do not, however, mean to represent 
it as preferable to the repetition of the observations; which 
process must, for many reasons, have the advantage over any 
mechanical contrivance whatsoever. 
I have procured Reflecting Circles to be constructed, upon 
the principles here described, both with the telescope and the 
horizon glass upon a moveable index, and with the same pieces 
attached to the main frame of the instrument. The two me- 
thods have respectively answered my expectation; and I pur- 
pose, at a future opportunity, to publish a description of the 
means which I wish to recommend for the mechanical improve- 
ment of the different parts, together with an account of some 
other essays which I have made relative to the same subject. 
