4 , 1 4* Afr. Pond on the different methods of 
expected. It appears to me that the former instrument must 
have described nearly a correct hour circle, though this circle 
was evidently not exactly the meridian. With all the decided 
superiority of the new instrument, I cannot venture to assign 
to the catalogue of my predecessor an error much greater than 
one tenth of a second of time, even in the stars near the horizon, 
where the error appears probably to have been the greatest. 
I trust it will be considered by astronomers, as creditable to 
the history of this Observatory, that two observers, with 
different instruments ( and by as different a method of com- 
putation as the case admits of ) should deduce two catalogues 
so exactly alike, that they may be considered almost as 
identical. 
It is an interesting question to every astronomer possessed 
of a valuable instrument, to know to what degree of accuracy 
its results can be depended on. I have examined a great 
number of observations made with the new transit instrument 
with this view, and it appears to me, that near the equator, 60 
observations will generally give the second decimal place of a 
second of time very correctly ; 120 observations will give this 
with greater certainty, but not in the proportion of two to one.* 
This I think is rather a greater exactness than can be obtained 
by the mural circle, f and the reason I apprehend to be this. 
* I find, by the rule given by M. Laplace, that the probable error of 120 obser- 
vations is o*,oo5 in time. 
+ The optical power of the new transit instrument is so decidedly greater than in 
the former one, that each observation must necessarily be more exact ; but I do not 
find the discordances in the ultimate results smaller in the same proportion : from 
this circumstance I conclude, that the limit to accuracy consists rather in the clock, 
than in the instrument. 
The Greenwich transit clock, compared with others, is, I believe, considered as a 
