caused by atmospherical Refraction, 245 



point, I had no means of measuring, with the accuracy that 

 would be requisite for determining the nature of the curves of 

 density formed according to the first proposition. 



But the truth of the second proposition appeared capable of 

 confirmation by experiment ; for the deviation of a ray is there 

 said to depend on the increment of density, and time of the 

 ray's passage, jointly ; accordingly, the deviation caused by a 

 given increment should be in proportion to the extent of the 

 medium. 



In order to try what effect a greater extent of medium would 

 produce, 



Exper. 3. I made a rectangular glass vessel, of which the 

 sides were in the ratio of 10 to 30,6; and, having put into it 

 some clear syrup, with water on its surface, I measured the 

 greatest refractions through it in both directions, and found them 

 in the ratio of 10 to about 29. 



In another vessel, of which the sides were as 10 to 40,4, the 

 refractions were, on an average, as 1 to 4. 



Being now fully satisfied of the effect of different fluids, I made 

 the following experiment, whereby it appears, .that the variations 

 of density occasioned by difference of temperature between ad- 

 jacent strata of the same fluid, follow a similar law. 



Exper. 4. Having put some cold water into a square vessel, 

 I covered its surface with a piece of writing paper perforated 

 with a few small holes, and then filled the vessel cautiously 

 with boiling water. The paper nearly prevented any mixture 

 of the hot and cold water; but, by floating gradually up, left 

 them to communicate their heat by contact alone. 



While they were in this state, I examined the appearance of 

 remote objects through the varied medium, and found, that 



