INTRODUCTION. 
Ixv 
u themfelves to our confideration. It will be fufficient to 
“ inftance the exceedingly fmall height to which the tide 
66 rifes, in the middle of the great Pacific Ocean ; where it 
a falls Ihort, two-thirds at leaft, of what might have been 
“ expe6led from theory and calculation. 
“ The direction and force of currents at fea, make alfo 
u an important objedt. Thefe voyages will be found to 
u contain much ufeful information on this head; as well 
u relating to feas nearer home, and which, in confequence, 
“ are navigated every day, as to thole which are more re- 
u mote, but where, notwithftanding, the knowledge of 
u thefe things may be of great fervice to thofe who are 
“ deftined to navigate them hereafter. To this head alfo 
“ we may refer the great number of experiments which 
ce have been made for inquiring into the depth of the fea, 
“ its temperature, and faltnefs at different depths, and in a 
u variety of places and climates. 
66 An extenfive foundation has alfo been laid for improve- 
“ ments in magnetifm, for difcovering the caufe and nature 
“ of the polarity of the needle, and a theory of its varia- 
“ tions, by the number and variety of the obfervations and 
“ experiments which have been made, both on the variation 
a and dip, in almoft all parts of the world. Experiments alfo 
“ have been made, in confequence of the late voyages, on 
“ the effe6ls of gravity, in different and very diftant places, 
“ which may ferve to increafe our flock of natural know- 
u ledge. From the fame fource of information we have 
46 learned, that the phenomenon, ufually called the aurora 
“ borealts , is not peculiar to high Northern latitudes, but 
“ belongs, equally, to all cold climates, whether they be 
66 North or South. 
“ Rut, perhaps, no part of knowledge has been fo great 
I* i 66 a gainer 
