PRELIMINARY REMARKS. 163 



obliquity of the earth's axis causes the changes of the seasons, and 

 the inequality of the days and nights. At the equator, the days 

 and nights are of equal length throughout the year ; but in all other 

 parts of the earth there is an inequality, which continually increases 

 as we approach the poles. In latitude. 41 21' the longest day is 15 

 hours; in latitude 58 25', it is 18 hours; at the polar circles it is 

 24 hours ; and at each pole the sun continues above the horizon six 

 months at a time, appearing to describe a horizontal circle, every 

 day, and never rising higher than 23 28'. 



The inequality of the sun's heat, in different parts of the earth, 

 causes the difference of Climates ; as approximately indicated by 

 the different zones. The torrid zone lies between the tropics ; the 

 temperate zones extend from the tropics to the polar circles, being 

 designated as the northern and southern ; and the frigid zones extend 

 from the polar circles to the poles. The climate of the Torrid zone 

 is generally hot ; that of the Temperate zones, warm or variable ; 

 and that of the Frigid zones, intensely cold. Mountainous regions 

 are always colder than low ones ; and even at the equator, at the 

 height of three miles above the level of the sea, there is perpetual 

 snow. The animal and vegetable productions of the earth, vary 

 with the climate ; having reference, of course, to the altitude of each 

 locality. The smaller and more hardy animals and plants are gene- 

 rally found in the colder regions ; and the larger animals and plants, 

 including the more venomous and poisonous, are found chiefly in the 

 torrid zone. Of these, however, and of the mineral productions, and 

 internal structure of the earth, we must defer further notice, to the 

 department of Idiophysics. 



The tides in the ocean, are chiefly caused by the moon's attrac- 

 tion, as it revolves around the earth; but partly by the attraction of 

 the sun. Similar tides doubtless exist in the atmosphere, or vestment 

 of air, which surrounds the earth, extending to the height of 40 or 45 

 miles on every side. The tides follow the moon's apparent diurnal 

 motion from east to west ; and are generally highest at any place, 

 about two hours after the moon has crossed the meridian ; there being 

 a high tide directly on the opposite side of the earth at the same time : 

 but, by the obstructions which the land presents, they are subjected 

 to great irregularities. When the tide is rising at any place, it is 

 said to be flood-tide ; but when it is falling, it is called ebb-tide ; 

 there being two flood tides, and two ebbs in about 25 hours ; and 

 the extremes of flood and ebb being called high and low water. The 

 highest tide in the world, is at Cumberland Head, in the Bay of 

 Fundy, where the greatest difference between high and low water, at 

 spring tides, when the attractions of the sun and moon are united, 

 is 71 feet. 



The air, when heated, expands and rises ; while colder air rushes 

 in below, to supply its place ; thus producing winds, or currents of 

 air. The trade-winds extend about 30 on each side of the equator, 

 and blow towards it, because it is in the warmest region ; but they also 

 incline westward, and, along the equator, blow almost directly from 

 the east, because they come from those parts of the earth that are not 

 moving eastward so fast as the equator is, by the earth's diurnal 



