50 
Mr. Pond on the changes in the 
In illustration of the whole of the preceding observations 
let us examine two catalogues, those of Dr. Brinkley, and 
Mr. Bessel, which have lately much excited the attention of 
astronomers. It is obvious, by merely inspecting these cata- 
logues, a comparison of which with the Greenwich catalogue 
I here subjoin, that one, or both, of the instruments used by 
these astronomers must be erroneous ; and it seems to me, 
that the source of error is the very flexure, the nature and 
effects of which we have been considering. For if we at- 
tend to the differences between these two catalogues, we 
shall find, that the six stars near the equator differ 5" from 
one another, whereas the stars near the zenith do not differ 
above 2", 5 . In which direction flexure will effect the zenith 
distances, is a matter quite accidental, depending on the 
unequal elevation or depression of the object-end or eye-end 
of the telescope, in consequence of the unequal strength of 
the materials. If we suppose error to exist in each of the 
catalogues, this cause must have had an opposite influence 
in the two cases : if we compare the Greenwich observa- 
tions with those of Dr. Brinkley, we shall arrive at the 
same conclusion ; namely, that the differences must be caused 
by flexure in one or both of the instruments ; since here 
also we find that the stars in the neighbourhood of the 
zenith are affected by only half the difference in polar dis- 
tance, that is observed in the stars near the equator ; and the 
same conclusions may be drawn from comparing the Green- 
wich observations with those of Mr. Bessel. The polar dis- 
tances of all the stars in Mr. Bessel’s catalogue exceed the 
instruments turning in azimuth, the advantage of measuring the double of the 
required angle. 
