150 LETTERS ON NATUEAL MAGIC. 



towards the centre. The very same effects are 

 produced in the air, only a greater tract of air is 

 necessary for showing the effect produced, by 

 heating and cooling it unequally. If we now 

 remove the lenses a, >, and hold a heated iron 

 horizontally above the water in the trough ABC, 

 the heat will gradually descend, expanding or 

 rendering rarer the upper portions of the fluid. 

 If, when the heat has reached within a little of 

 the bottom, we look through the trough at the 

 ship S in the direction ES', we shall see an in- 

 verted image at S', and an erect one at S' ; and 

 if we hide from the eye at E all the ship S, ex- 

 cepting the top-mast, 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 trans- 

 parent 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 frequently 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 



