PHILOSOPHICAL 
TRANSACTIONS. 
I. The Bakerian Lecture . Observations on the Quantity of hori- 
zontal Refraction ; zvith a Method of measuring the Dip at Sea. 
By William Hyde Wollaston, M. D. F. R. S. 
Read November 11, 1802. 
In a Paper which I some time since presented to this Society, 
(printed in the Phil. Trans, for 1800,) I endeavoured to ascer- 
tain the causes, and to explain the various cases, of horizontal 
refraction, which I had either observed myself, or had seen 
described by others. 
At the time of writing that essay, I had not met with the 
Memoires sur I’Egypte, published but a short time before; and 
I was not aware that an account had been given by M. Monge, 
of the phenomenon known to the French by the name of 
mirage , which their army had daily opportunities of seeing, in 
their march through the deserts of Egypt. 
In the perusal of this memoir, I could not fail to derive 
instruction from the information it contained ; but, as the facts 
related by him accord entirely with the theory that I had 
advanced, I was by no means induced to adopt the explanation 
that he has proposed, in preference to my own. 
MDCCCIII. B 
