EXPLANATION OF THE DIP SECTOR,
REMARKS ON TJÍE OBSERVATIONS MADE WITH IT IN HIS
MAJESTY’S SLOOP LYRA.
I n our tables for apparent dip of the visible horizon at
different heights from the sea, as calculated from the known
curvature of the earth, allowance is made for the refraction
ofj the atmosphere, on a supposition of its being constant,
but as.it is. known to vary, the tabular dip will often.be erroneous,
and, consequently, altitudes taken, under, different:
states of the atmosphere, will exhibit different instead of
corresponding results.
I t is foreign to the present purpose to shew what the
causes are which have most, effect in raising or depressing
the apparent horizon. I t may be sufficient to mention,
that changes in. the relative temperature of the air and the
sea must produce changes in the refraction-near the surface..
Dr. Wollaston has published two papers in the Philosophical
Transactions on this subject, in the volumes for 1800 and
1803, and to these I beg to refer the reader for precise information
upon this very curious subject.
The object which this sector proposes to attain, is the
actual admeasurement of the dip angle; that is, to ascertain
how much the visible horizon is depressed below the horizontal
plane passing through the eye of the observer.. The