with a Method of measuring the Dip at Sea. g 
the thermometer or hygrometer, yet the law of these variations 
is not altogether so simple as I had hoped it might be found. 
I shall, on the present occasion, first relate the facts on which 
this opinion is founded, and which are in themselves sufficiently 
remarkable, on account of the unexpected quantity of refraction 
observable over a short extent of water; I shall, in the next 
place, shew that the exact determination of the concurrent 
changes of the atmosphere are of less value, and their irregu- 
larities of less consequence, than I had conceived, as there is a 
very easy method whereby the quantity of dip at sea may be at 
any time correctly measured; and therefore the end which I 
sought by indirect means, may be at once directly attained. 
The first instance that occurred to me, of observable refraction 
over the surface of the Thames, was wholly accidental. I was 
sitting in a boat near Chelsea, in such a position that my eye 
was elevated about half a yard from the surface of the water, 
and had a view over its surface, that probably somewhat ex- 
ceeded a mile in length, when I remarked that the oars of 
several barges at a distance, that were then coming up with the 
tide, appeared bent in various degrees, according to their dis- 
tance from me. The most distant appeared nearly in the form 
here represented; dd being my visible horizon by apparent 
curvature of the water; ah the oaritselfln its inclined position; 
and he an inverted image of the portion be. By a little attention 
to other boats, and to buildings on shore, I could discern that the 
B 2 
