218 LETTERS ON NATURAL MAGIC. 



the ship S excepting the topmast, we shall have an exact 

 representation of the phenomenon in Fig. 33, The 

 experiment will succeed better with oil in place of water ; 

 and the same result may be obtained without heat, by 

 pouring clear syrup into the glass trough till it is nearly 

 one-third full, and then filling it up with water. The 

 water will gradually incorporate with the syrup, and 

 produce, as Dr. Wollaston has shown, a regular gradation 

 of density, diminishing from that of the pure syrup to 

 that of the pure water. Similar effects may be obtained 

 by using masses of transparent solids, such as glass, rock 

 salt, &c. 



Now it is easy to conceive how the changes of density 

 which we can thus produce artificially may be produced 

 in nature. If in serene weather the surface of the sea is 

 much colder than the air of the atmosphere, as it fre- 

 quently is, and as it was to a very great degree during the 

 phenomena described by Mr. Scoresby, the air next the 

 sea will gradually become colder and colder, by giving 

 out its heat to the water ; and the air immediately above 

 will give out its heat to the cooler air immediately below 

 it, so that the air from the surface of the sea, to a con- 

 siderable height upwards, will gradually diminish in 

 density, and therefore must produce the very phenomena 

 we have described. 



The phenomenon of Dover Castle, seen on the Eamsgate 

 side of the hill, was produced by the air being more dense 

 near the ground, and above the sea, than at greater heights, 

 and hence the rays proceeding from the castle reached the 

 eye in curved lines, and the cause of its occupying its 

 natural position on the hill, and not being seen in the air, 

 was that the top of the hill itself, in consequence of being 

 so near the castle, suffered the same change from the 

 varying density of the air, and therefore the castle and 

 the hill were equally elevated and retained their relative 



