Proceedings of the Polytechnic Association. 713 



beach parties of snipes and sand pipers scamper in pursuit of their 

 prey, wliich is washed up in the rolls of sea-weed by the little waves. 

 Kow and then, as a boat passes, yellow water-snakes will suddenly 

 erect their heads and show their fangs with an angry hissing. Occa- 

 sionally, shoals of grampus enliven the scene, splashing, leaping and 

 hunting one another with the greatest liveliness. The white, calm 

 bay, with its background of rich evergreen foliage, and the light 

 feathery clouds drifting over with the steady trade wind, form a coup 

 (Tmil only to be imagined in the dark and stormy north." 



MlKAGE. 



The rare optical phenomenon known as mirage consists in the 

 apparent displacement of objects in the vicinity of broad sheets of 

 water or over sandy plains, and is due to the refraction of light. 

 Air in contact with a heated portion of the earth's surface becomes 

 dilated ; therefore in sustaining the weight of the incumbent atmos- 

 phere its elasticity is increased, while its density is diminished. 

 Adjacent layers of air of different densities have diiferent refracting 

 powers, and rays of light coming with great obliquity from a distant 

 object before reaching the earth are thus bent upward, and present 

 to the observer the impression of light reflected from the surface of 

 water ; and where such object is over water, both the light directly 

 from it and that of its reflection reach the eye at the same instant, 

 by which two images are seen opposite one to the other, and joined 

 at their bases. This class of phenomena must not be confounded 

 with that due to the reflection of terrestrial objects on the clouds, 

 distinguished as spectra. Mirage is visible near the horizon, and when 

 the object is not far off is seen more distinctly as the eye approaches 

 the ground. Many details of mirage which escape the naked eye 

 may be revealed by the telescope. The contrasts of temperature 

 producing mirage occur frequently in the artic regions when the 

 summer sun acts powerfully on masses of ice ; in milder climates 

 such contrasts are rare. When strata of air of different density 

 extend vertically, instead of horizontally, by means of strong currents, 

 it would be inferred that the object will appear at right angles to its 

 true position, and such has been proved by observation to be the fact. 

 Lateral mirages have been seen on Lake Geneva, Switzerland. In 

 these phenomena of refraction, the distance from the observer to the 

 actual position of the objects represented seldom exceed ten or fifteen 

 miles. A very remarkable case occurred on the loth of April, at 



