212 Captain Hall’s details of experiments 
The methods followed for making the adjustments of the 
instruments, conducting the experiments, and deducing the 
results, were those laid down in your paper on the length of 
the pendulum at the principal stations of the Trigonometrical 
Survey. I took care, for example, always to adjust the dia- 
phragm in the focus of the eye-piece of the telescope, so that 
its edges should coincide exactly with those of the extremity 
of the pendulum ; according to the precept at page 9 of your 
second paper, (read before the Royal Society in June, 1819.) 
This adjustment, by the way, is rendered more easy and exact, 
by placing a card or other white object at a little distance 
behind the pendulum. I also invariably determined the 
Intervals by observing the disappearance of the white disk 
according to your directions at page 1 1 , and the reasoning at 
page 58 of the first paper,* (read in January, 1818.) 
And here I feel it not only due to you, but likely, perhaps, 
to be of use to future observers, to state that, after many trials 
of fancied improvements, and simplifications of your methods, 
both in the conduct of the experiment itself, and in the subse- 
quent computations, I was finally obliged to acknowledge, 
in every instance, even where I succeeded, that I had by 
more labour, or by a more circuitous path, reached the same 
point to which your rules would at once have led me. 
* I am particular in stating these two circumstances, especially the first, from its 
being so essential to the accuracy of the whole experiment, in all cases where the 
diameter of the disk and the breadth of the pendulum, though in fact equal, are placed 
at different distances from the eye, and therefore appear under different angles ; and 
not, as in your first paper, where they are so proportioned that both occupy the 
same apparent space when seen through the telescope. I was at first disposed to 
think it might be better to observe both the times of disappearance and re-appear- 
ance of the white disk, and to assume the mean as the true instant of the coincidence ; 
but I found by repeated trials, that the time of re-appearance was liable to greater or 
less uncertainty according to the degree of light, and other unmanageable circum- 
stances : and having satisfied myself by demonstration that the method of obtaining 
the intervals by observing the disappearance was rigorously correct in principle, I 
adhered to it ever afterwards as being more simple and infallible in practice. 
