9 
with a Method of measuring the Dip at Sea. 
micrometer, and consequently are not liable to any error from 
unsteadiness of the instrument or object. 
From the foregoing observations we learn, that the quantity 
of refraction over the surface of water may be very considerable, 
where the land is near enough to influence the temperature of 
the air. At sea, however, so great differences of temperature 
cannot be expected; and the increase of dip caused by this 
variation of horizontal refraction, it is to be presumed, is not so 
great as in the confined course of a river ; but, if we consider 
that it may also be subject to an equal diminution from an op- 
posite cause, and that the hofizon may even become apparently 
elevated, there can be no question that the error in nautical 
observations, arising from a supposition that it is invariably 
according to the height of the observer, stands in need of 
correction. 
The remedy employed by Mr. Huddart,* of taking two 
angles of the sun from opposite points of the horizon at the 
same time, and considering the excess of their sum above i8o a 
as double the dip, must without doubt be effectual ; but, from 
causes which he assigns, it is practicable only within certain 
limits of zenith distance; for, where the zenith distance is small, 
and the changes of azimuth rapid, there is required considerable 
dexterity and steadiness of a single observer who attempts to 
turn in due time, from one observation to another ; and, when 
it exceeds 30°, the greater angle cannot be measured with a 
sextant, and consequently his method is, with that instrument, 
of use only in low latitudes. 
On account of the difficulty attending some of the adjust- 
ments for the back observation, he rejects that method for 
* Phil. Trans, for 1797, p. 40. 
c 
MDCCCIII. 
