experiments made with an invariable pendulum. 13 
The experiments above described are of a nature to require, 
at every stage, the utmost degree of care ; since an error, 
very small in apparent amount, either in the observations 
themselves, or in the subsequent computations, may prove 
fatal to that minute accuracy, without a due attention to 
which the nice objects of this problem might easily elude 
our notice. 
It will readily be understood, therefore, by every one con- 
versant with such undertakings, that the observer, besides 
possessing adequate leisure, must be duly assisted in all parts 
of his progress by those persons with whom he is associated. 
And as it has been my good fortune to meet not only with 
the heartiest encouragement, but also the most efficient co- 
operation from the Commander of the Expedition, throughout 
the whole course of these and various other delicate re- 
searches, I feel it my duty not less on public grounds, than 
as a matter of private respect and gratitude, to make this 
acknowledgment of the source, to which every thing that 
may appear valuable in these enquiries is justly to be traced. 
HENRY FOSTER. 
