I N T R O D U C T I O No Ixv 



" themfelves to our confideration. It will be fufficient to 

 *' inftance the exceedingly fmall height to which the tide 

 * rifes, in the middle of the great Pacific Ocean ; where it 

 ^ falls fliort, two-thirds at leaft, of what might have been 

 '• expected from theory and calculation. 



*' The direction and force of currents at fea, make alfo 

 ** an important objecSt. Thefe voyages will be found to 

 " contain much ufeful information on this head ; as well 

 " relating to feas nearer home, and which, in confequence, 

 '* are navigated every day, as to thofe which are more re- 

 *' mote, but where, notwithftanding, the knowledge of 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 have been made 

 *« for inquiring into the depth of the fea, its temperature 

 " and faltnefs at different depths, and in a variety of places 

 « and climates. 



" 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 

 *' and dip, in almoft all parts of the world. Experiments alfo 

 " have been made, in confequence of the late voyages, on 

 *' the effects of gravity, in different and very diflant places, 

 " which may ferve to increafe our flock of natural know- 

 ** ledge. From the fame fource of information we have 

 " learned, that the phsenomenon, ufually called the aurora 

 " borealis, is not peculiar to high Northern latitudes, but be- 

 *' longs, equally, to all cold climates, whether they be North 

 " or South. 



Vol. L i *« But, 



