with a Method of measuring the Dip at Sea. xi 
method, is the possibility that the refraction may be in some 
measure different in opposite points of the horizon at the same 
time. When land is at no great distance, such an inequality 
may be found to occur ; but, upon the surface of the ocean in 
general, any partial variations of temperature can rarely be 
supposed to exist; and it is probable, that under any circum- 
stances, the difference will not bear any considerable proportion 
to the whole refraction; nor can it be thought a sufficient 
reason for rejecting one correction proposed, that there may 
yet remain other smaller errors, to which all methods are equally 
liable, but which it is not the object of the present dissertation 
to rectify. 
