6 Dr. Wollaston’s Observations on horizontal Refraction ; 
over the course of the river. This circumstance deprived me of 
many opportunities which the changes of the thermometer 
indicated to be favourable for my purpose,and obliged me often 
to make use of shorter distances than I should otherwise have 
chosen, by bringing the line of sight as near as I could to the 
surface of the water. 
For this purpose, I had a plane reflector fitted to the object- 
end of a small pocket telescope, at an angle of 45 0 , so that, 
when the telescope was held vertically, it gave a horizontal view 
at any level that was found most eligible. When the water has 
been calm, I have observed that the greatest refraction was 
visible within an inch or two of its surface, and I have then 
seen a refraction of six or seven minutes in the space of 300 or 
400 yards : at other times, I have found it greatest at the height 
of a foot or two ; but, in this case, a far more extensive view 
becomes necessary. 
The first measures that I took were on the 23d of September, 
1800. The water was 2-§-° warmer than the air, and I found a 
refraction of about 4'. 
Oct. 17. The difference of temperature was 3 0 , and the 
refraction 3'. 
Oct. 22. The water was 11 warmer than the air, yet the 
quantity of refraction did not exceed 3'. 
The smallness of the quantity of refraction upon this occasion, 
I attributed to the dryness of the atmosphere, conjecturing that 
a rapid evaporation might in great measure counteract that 
warmth which the water would otherwise have communicated 
to the air. 
From that time, therefore, I have noted not only the heights 
of the thermometer in the water and in the air, but have added 
