THE SEA AND THE ATMOSPHERE. 13 



simply rise and fall, in the open ocean, to an elevation of two or 

 tliree feet in all ; along onr shores, and np gulfs and estuaries, 

 they sweep with the violence of a torrent, having a general 

 range of ten or twelve feet — sometimes, as at Fundy, in Ame- 

 rica, at Brest and Milford Haven, in Europe, to a height of from 

 forty to sixty feet. The tides sweep our shores from filth, and 

 jnirify our rivers and inlets, affording to the residents of our 

 islands and continents the benefits of a bi-diurnal ablution, and 

 giving a health, and freshness, and purity wherever they appear. 

 Obedient to the influence of bodies many millions of miles re- 

 moved from them, their subjection is not the less complete ; the 

 vast volume of water, capable of crushing by its weight the most 

 stupendous barriers that can be opposed to it, and bearing on its 

 bosom the navies of the world, impetuously rushing against our 

 shores, gently stops at a given line, and flows back again to its 

 place when the word goes forth, ' Thus far shalt thou go, and no 

 farther ;' and that which no human power or contrivance could 

 have repelled, returns at its appointed time so regularly and 

 surely that the hour of its approach, and measure of its mass, 

 may be predicted with unerring certainty centuries beforehand. 



37. Hurricanes. — "The hurricanes which whirl with such 

 fearful violence over the surface, raising the waters of the sea 

 to enormous elevations, and submerging coasts and islands, 

 attended as they are b}' the fearful attributes of thunder and 

 deluges of rain, seem requisite to deflagrate the noxious gases 

 which have accumulated, to commingle in one healthful mass 

 the polluted elements of the air, and restore it fitted for the ends 

 designed for it. We have hitherto dealt with the sea and air — 

 the one the medium through which the commerce of all nations 

 is transported, the other the means by which it is moved along 

 — as themselves the great vehicles of moisture, heat, and cold 

 throughout the regions of the world — the means of securing the 

 interchange of these inestimable commodities, so that excess may 

 be removed to where deficiency exists, deficiency substituted for 

 excess, to the unbounded advantage of all. This group of illus- 

 trations has been selected because they are the most obvious, 

 the most simple, and the most intelligible and beautiful that 

 could be chosen. 



38. Powers of the air. — "We have already said that the atmo 

 sphere forms a spherical shell, surrounding the earth to a depth 

 which is unknown to us, by reason of its growing tenuity, as it 



