152 Mr. Wollaston's Description 
then set the lower limb to the upper wire, and read that off ; 
set the upper limb to the lower wire ; am ready to observe 
the second limb of the sun at the 3d, 4th, and 5th wires ; 
and lastly, read off the upper limb after the observation is 
ended. In this way, one has the meridian passage through 
the middle of the field, or within 2' of it : and the meridian 
altitude of both the limbs, while the sun's centre is on the 
meridian ; for the little alteration in altitude is soon done, and 
can disturb nothing. 
Indeed, upon the whole, this instrument itself is capable of 
doing a great deal of good work ; and convinces me fully, that 
one between piers would be highly advantageous to astro- 
nomy. As a transit, mine is perfect, so far as that size per- 
mits : indeed it is in fact to all intents a transit-instrument. 
And for altitudes ; since the readings are totally independent 
of the circle, though you have it in your power to re-examine 
your microscopes by the plumb-line between each observa- 
tion, if you please ; you find there is no occasion for it. In that 
respect, it has the advantage over a quadrant. No force is 
used in setting this instrument : the whole, from its form, 
is counterpoised in itself : there is no more probability of de- 
ranging it in altitude, than in azimuth : and therefore, all 
you have to do in actual observation beyond a common 
transit-instrument, is, to bisect the star as it passes, or as 
soon as ever it has passed the meridian wire, and read off 
the microscopes afterwards. Thus every observation is com- 
plete; by ascertaining the right ascension and altitude of 
every object at once, and with very little trouble ; which 
must tend greatly to the improvement of our catalogues. 
There is one additional advantage in an instrument of 
