$4$ Mr. Wollaston's Description 
grees, by just double that quantity. Hence, the difference 
from go degrees, at the same time that it gives a mean be- 
tween the two readings, would reduce the error or deviation 
of the axis to nothing. 
The instrument here described, is of the size I thought 
would be most convenient for my own use : indeed it is full 
as large as I should recommend ever to be made in that 
moveable form. It stands on a cylinder of one sofd stone, of 
2,y§- inches diameter, and 3 feet 6 inches long, bedded on a 
pier of brick, well bonded together, and rising from a good 
foundation, deep in the earth. The stone is clear of the floor 
all round, and is very steady indeed : the instrument rarely 
varies at all, in any respect. It is adjusted in the meridian 
to two marks, the one north, the other south ; so that now 
they are truly placed, the collimation of the telescope is 
easily examined, without lifting the circle out of its Ys. 
I may be supposed partial to an idea which I have long 
entertained ; but I confess, I should very strongly recom- 
mend the having an instrument of this nature, though more 
perfect, in every observatory ; I mean a transit instrument, 
on stone piers, with a suitable circle and microscopes ; that, 
whenever you observe a meridian passage, you may, at the 
same time, measure the exact altitude, or zenith distance of 
every object seen. The being obliged, in the common way, 
to have recourse to two different instruments, occasions the 
zenith distances to be much less frequently observed, than it 
is to be wished they were. It is true the British catalogue 
was, for the most part, deduced from observations with a 
quadrant alone ; and so was Mayer's. But, though labour 
and patient perseverance, may enable an observer to allow for 
