36 



INTRODUCTION. 



Fat:i Alurrrana. 



14. Mirage. The mirage, oi 

 appearance of objects which are 

 not actually in the horizon, or 

 which exist there in a difl'erent 

 situation, is one of ihe iiiost re- 

 markable of optical illusions. At 

 sea, rocks and sands concealed 

 under the water, appear as if they 

 were raised above the surface. 

 The Swedisli sailors long search- 

 ed for a pretended magic island, 

 which from time to time could be 

 descried between the isle of Aland 

 and the coast of Upland. It 

 proved to be a rock, the image of 

 which was presented in the air by 

 the mirage. At one time the 

 English saw with terror the coast 

 of Calais and Bologne, in France, 

 But the most celebrated example of mirage 

 The inhabitants, standing on the Italian side. 



apparently approaching the shores of their island, 

 is frequently exhibited in the straits of Messina. 



perceive images of palaces, embattled ramparts, houses, and ships, and all the varied objects 

 of towns and landscapes, in the air. This appearance is regarded by the people as the work 

 of fairies, and is called Fata Morgana. 



15. Aurora Borealis. This phenomenon is 

 commonly called the northern lights ; it is fre- 

 quently to be seen in New England, and generally 

 in the north. It usually commences two or three 

 hours after sunset, and consists of a whitish light, 

 sometimes appearing and disappearing, and occa- 

 sionally flashing nearly up to the zenith. Pre- 

 vious to the war of the Revolution, these hghts 

 assumed a reddish hue, and were imagined to 

 represent, in their bloody outlines, the marching 

 of armies and the turmoil of battle. In more 

 northern regions they ofJer a more brilliant dis- 

 play than in our country. Various explanations 

 are given of these remarkable phenomena ; but 

 n r ■ T 1 I there is no estabhshed theory on the subject. 



.'lurora Borealis, in Lapland- J •> 



VIT. STRUCTURE OF THE EARTH. MINERALS. 



1 . Arrangement of the Jllalerials. Persons unaccustomed to examine the structure of the 

 crust of the earth, are apt to imagine that mineral masses present a confused heap of matter, 

 without any particular order or arrangement. But wherever water-courses, artificial excava- 

 tions, or precipices, expose the structure, we find striking proofs of the agency of causes that 

 must have acted with great uniformity over vast portions of the surface, and that have produced 

 a general resemblance between the structure of widely distant countries. Much of the materials 

 is arranged in beds, of different extent and thickness, but indicating the operation of one com- 

 mon agent. Such beds are termed strata., and the general fact is expressed by the term strati- 

 fication. These strata sometimes consist of loose or slightlv consolidated masses of clay or 

 sand, and sometimes of hard, stony bodies, which are easily split .n ttie direction of the strata. 

 The schists or slates are an example. Wherever natural or artificial sections lay open the 

 earth's crust to a sufiicient depth, w e generally find the strata resting on a different kind of rock, 

 which has no marks of a stratified structure. Hence the distribution of rocks into two great 

 divisions, the stratified and the unstratified ; examples of the latter are granite, quartz, &c. 

 All of the unstratified and some of the stratified rocks have a granular or ciystaline structure, 

 whi'h indicate that they liave formerlv been melted by the action of fire : while the regular 



